Additive fabrication technology started out nearly twenty years ago being called "rapid prototyping" (RP). At the time, that was fairly descriptive of what it did. Early machines were used to make things that represented the general physical shape of some final part or other items. Since these parts or objects didn't have the material properties nor probably the accuracy that were required for actual use, they were just prototypes - examples, if you will. You couldn't do much with them, but engineers could pass them around at a meeting and say to one another things like, "Why, yes Fred, you seem to have left off the tubulation on the forambulater and made the trailing bushings too long. Please pass me a donut."

The next meeting would find the corrections added. Maybe another prototype was made, possibly even many - but also maybe not. One prototype might be all that was required. Seeing and feeling something in the approximate form it would ultimately take turns out to be a very good way of short-circuiting the process of going from an idea on paper or a screen, to making something that actually works.

This was one reason why the process of design became faster and put the rapid in "rapid prototyping," but it wasn't the only one. Instead of having to cut a material away to sculpt a part, you could just give a computer file to the machine and it could spit out even a very complicated part fairly quickly. Quickly is relative: Sometimes it took days to make a part, and often it still does. But it might take much longer to make the same thing by cutting material away using subtractive fabrication. So these additive processes were rapid in comparison to that, and that made for another reason to call them "rapid."

Outrageous Interference
Of course, whoever started using the term "rapid prototyping" for these additive fabrication technologies twenty years ago, either didn't know, or didn't care, that others in completely unrelated fields were using it, as well. Some computer programming techniques are called rapid prototyping, as are ways to quickly generate web-sites. Individuals designing advanced control systems for aircraft and similar applications sometimes describe the process as rapid prototyping. Biologists rapid prototype microfluidic systems using a variety of methods such as stamp lithography. Sometimes it isn't easy to tell who's doing rapid prototyping and who's doing additive fabrication.

Confused usage
The name of one technology in particular - stereolithography - is sometimes used as a synonym for "rapid prototyping." A manufacturer of stereolithography equipment might like this a lot. If prospects think rapid prototyping is stereolithography - even though that's only one flavor of additive fabrication - that might be advantageous. This usage seems most prevalent among individuals that are learning about the field for the first time, however.

Confusion among the synonyms
Along the way "rapid prototyping" has been called by several additional pet names provided by corporations, writers and the academic community. Here are some that you'll see from time to time:

Desktop manufacturing (DTM) - This was an early term and from which one of the early companies in the field derived its name (DTM, Inc.). Unfortunately, the only "desk" that one of these machines in the early days could have fit on was something like a Nimitz-class aircraft carrier. Only in recent years have any additive fabrication machines been able to fit on anything resembling furniture. There will very likely always be additive fabrication machines that won't fit on desks limiting the generality of the term. DTM doesn't come up often, but it still appears once in a while.

Solid freeform fabrication (SFF) / freeform fabrication (FFF) - The academic community likes these very much. However, the words don't sing and are too much of a mouthful to lead to great popularity. Nevertheless, they're often used in academic papers and patents, and aren't going to go away soon.

Solid imaging / solid imager - 3D Systems and some Japanese companies have used these terms, often applying them specifically to stereolithography, but sometimes meaning the entire field. Solid imaging seems to have fallen by the wayside in recent years, but is still seen once in a while.

Fabber / fabbing - Author Marshall Burns has promoted this term. When he was in academia, we observed his students freely engaging in its use. It leaves open the possibility of an X-acto knife or a milling machine being called a fabber, which they are. In addition, the term is far too redolent of the decade of the sixties. Using the term "fabber" will lead to wanton interpersonal acts, distortions of reality and very loud music.

Layered manufacturing / layered fabrication - All of the commercial systems do indeed operate by making layers and bonding them - some thicker, some thinner. But additive fabrication systems need not do so by definition. There are other ways to add materials together to make things that have been described in the literature. For example, one could build an object in a cylindrical fashion. It's not likely that other such geometries will be of much commercial interest soon, so layered fabrication is not so bad. The term seems mostly to come up in trade magazines when writers are explaining the field to an unfamiliar audience.

New capabilities, but OLD terminology
The next thing that inevitably happened was that materials and accuracy improved, so rapid prototyping could be used to make functional parts, as well as ones you just looked at. Now, not only was rapid prototyping not very rapid - it wasn't necessarily making prototypes either. There are, however, such things as functional prototypes, and even though some parts may have been used in final applications, "rapid prototyping" still wasn't a complete misnomer at this point. Besides, old habits are hard to break.

Lower prices, but NEW terminology
Some rapid prototyping machines could still not make functional parts and these were called "concept modelers" once others machines could. These technologies were only appropriate for the original application of rapid prototyping, to check the form of something. The term remained current for a few years, and is still seen once in a while.

Then some additive fabrication machines got much less expensive. To differentiate this segment of the market from more costly machines, people started calling these machines "3D printers" or "three dimensional printers." Now, there already was a specific technology with a very similar name called Three Dimensional Printing (3DP) (TM) which had been developed and trademarked by MIT. One of its licensees, Z Corporation, actually makes 3D printers using Three Dimensional Printing (TM) technology. But other manufacturers make 3D printers using other technologies. For example, Stratasys makes 3D printers using fused deposition modeling (FDM).

Surely, there must be some additional difference between 3D printers and rapid prototyping machines? No; just the price and maybe some specifications. The specifications keep getting better for so-called 3D printers and their prices keep getting lower. So if you base a distinction in terminology on those features today, there's a good chance that you will be wrong tomorrow. For example, a Lexus is a type of automobile and so is a Corolla. One is not quite the same as the other, and one costs much more - but either one will get you where you're going.

Rapid tooling arises from rapid prototyping
Rapid prototyping started out by making parts mostly out of plastic in one form or another. It wasn't very long before some of the technologies were extended to making things out of metals and other more durable substances, as well. That meant that rapid prototyping could then be used to make complex tools like injection molds and die casting molds. The natural extension of the terminology was to call this rapid tooling (RT) - especially so since the speed of fabrication was a paramount reason for the existence of the application in the first place.

A number of rapid prototyping technologies, such as selective laser sintering and laser powder forming, can be used to make tools directly in metals and other materials. Applying the term "rapid tooling" to these processes in such applications seemed quite correct.

But other additive fabrication methods could be used to make a pattern - or model - from which a tool could also be made using a secondary process. Some of these so-called indirect methods deposit a final material for the tool such as metal right on the pattern. When the pattern is removed, the tool is largely complete. In other cases, these added steps involve a material transfer process. The pattern may be first used to make a mold and a harder final material for the tool is formed from that. Several such transfer steps may be required depending on the particular process.

Sometimes these material transfer processes themselves have also been called "rapid tooling." This is not precisely correct. Many of them existed long before additive fabrication was ever thought of, and virtually all of them can utilize patterns made in any fashion, whether using additive technology or not. There are many such processes. Among them are; RTV silicone rubber molding, aluminum-filled epoxy tooling, KelTool(TM), EcoTool(TM) and others. They can use patterns made by additive fabrication technologies - but they could also use patterns made by hand, or even shells or stones from a beach for that matter.

Therefore, to be very precise - OK, fussy - a material transfer process by itself doesn't qualify as "rapid tooling." However, using one of them in conjunction with a rapid prototyping-generated pattern does, and these combined processes are properly classified as "indirect methods" of rapid tooling.

Not just rapid tooling
Some additive fabrication technologies that are mostly used for rapid tooling applications are described as rapid tooling technologies. This is also incorrect because they can, and often are, used to make other things besides tooling. For example, laser powder forming technologies are frequently used to make injection mold tooling, but they can also be used to make hip implants.

Among the methods that are often thought of as "rapid tooling" technologies are: Direct Metal Deposition (TM), ProMetal (TM), electron beam melting (EBM), selective laser melting (SLM) and others. Actually, they're just additive fabrication processes that are often used to make tools.

Rapid manufacturing arises from rapid prototyping
Once rapid prototyping materials and accuracy improved, it became possible to make functional parts that could be used in final applications. So-called "mass customization" consequently arose as a new possibility - manufacturing a product for a limited number of customers, or even for just one individual. While we're not used to thinking of manufacturing with a lot size of one, it's not hard at all to find applications where only a few of something are needed. Additive processes are turning out to be good prospects for many of those applications, especially ones that involve complex shapes.

The term "rapid manufacturing" arose in a natural way, and at least partially as a way to differentiate it from rapid tooling. It describes how rapid prototyping avoids the usually-required lengthy and expensive process of making a manufacturing tool. Of course, just as rapid prototyping is not necessarily rapid, rapid manufacturing can be quite slow, too. In some cases it's very slow indeed.

Synonyms in waiting
Just as with rapid prototyping, a number of synonyms for rapid manufacturing are used. None of them call it rapid, and all of them are now fighting it out in the literature for supremacy:

Additive fabrication / additive manufacturing - Additive fabrication is a somewhat more general term than additive manufacturing. It's probably best reserved as the umbrella term that subsumes all of the technologies and applications described herein. But both terms are used to mean rapid manufacturing and are often seen. Additive manufacturing is probably the most precise term for what is being done in rapid manufacturing. Not only that, but in early 2009 the ASTM placed its imprimatur on the term after a specially-called meeting of 70+ industry experts hotly debated the topic. Even better - almost no one has used "additive manufacturing" up to now, so it should be easy to see if the opinion of industry experts actually counts for anything.

Toolless manufacturing - This was an early term for rapid manufacturing, but has become less used recently, possibly because no one knows how to spell tooless, or whether tool-less should be hyphenated. Besides, no one wants to be toolless.

Digital manufacturing / direct digital manufacturing / digital fabrication / Advanced Digital Manufacturing (ADM) (TM) - The latter is a trademark of 3D systems, but the other variations are much more often seen. They're reasonably descriptive and place the emphasis on the fact that computation is the driving force. But then, computation is the driving force behind nearly everything in modern life and the terminology may not survive because it's insufficiently specific with respect to the technology.

Direct digital manufacturing (DDM) has fairly recently been given the blessing of the SME, which group (obviously a large committee) - quite interestingly - uses the term "additive manufacturing" to define it.

Direct fabrication / direct manufacturing - Not very often used, but the emphasis is on the fact that the use of manufacturing tooling is avoided.

When will it all end and agreement on terminology be reached?
It will never end and agreement will never be reached. Instead, users and the general public will slowly center on the terminology that makes the most sense for their use of the technology in particular applications. Certain nomenclature will emerge as clear winners. We predict:

The entire field will be known one day as additive fabrication.

Rapid manufacturing will become known as additive manufacturing, but additive fabrication will also frequently be used.

Rapid tooling will be most often associated with technologies that make tools directly, but confusion will continue.

Freestanding and desktop machines that are used to make prototypes, models and parts will be called 3D printers, regardless of the flavor of technology actually used.

Stereolithography will continue to be used as a synonym for additive fabrication, and will extend to other emerging photopolymer-based technologies, as well.

Rapid prototyping will slowly dwindle away, and eventually won't be used at all - because it's not really rapid and it does much more than make prototypes.