Technology Readiness Levels (TRLs) are a systematic
metric that provides an objective means by which to explain the maturity of a particular technology. They were originally developed by
NASA, but with a little modification (getting rid of all the "in space" qualifiers, for example), could be used to express the readiness level of just about any technology project.
For example: say I am a computer game developer, and my publisher wants to know how close to "done" we are. I could say, "Pretty close," but that has such ambiguous meaning. On the other hand, going into detail on the specific things we have finished, and the things that still need work may get too technical for our representative. If, however, I was able to say, "We are at TRL 6," then I have just summarized the status of the project, which is really all my publisher is asking. Any specific questions they may have can then be addressed with this out of the way.
Without further ado:
TRL 1: Basic principles observed and reported, or "Hey, that's neat."
This level represents pure research. There really isn't even a particular piece of technology in question. We might be studying basic properties of materials, or noticing what works really well in Warcraft III.
TRL 2: Technology concept and/or application formulated, or "Ooo, idea!"
This level represents taking our observations and coming up with some sort of practical use for them. Things are still speculative. We could be thinking about superconductors or Real-time strategy games.
TRL 3: Analytical and experimental critical function and/or characteristic proof-of-concept, or "Let's do it."
Development has begun. All we're trying to produce is proof-of-concept for the stuff we came up with in TRL 2. Getting an experimental process to work in a laboratory setting, for example.
TRL 4: Component and/or breadboard validation in laboratory environment, or "Gold spike!"
We take our proof-of-concepts from TRL 3, and we integrate them into a lo-fi version of the system we came up with in TRL 2. A playable demo for project-pitching purposes, for example.
TRL 5: Component and/or breadboard validation in relevant environment, or "Alpha"
Similar to TRL 4, but this version is robust enough to deal with "real life" conditions, or, at least, a decent simulation of those conditions. Testing something in a vacuum, or a playable demo that you could bring to a conference for people to try out.
TRL 6: System/subsystem model or prototype demonstration in a relevant environment (ground or space), or "Beta"
Any model or prototype is now well beyond the jerry-rigged TRL 4 version. At this point, testing is happening in a real environment. Beta testers are called in, or you throw it on a shuttle, and try it out in space. According to NASA, this step is driven more by management confidence than actual technical requirements.
TRL 7: System prototype demonstration in a space environment, or "Things! In! Spaaaace!"
Not mapping very well to projects outside of NASA, this level is for the purpose of assuring system engineering and development management confidence. Not all technologies need this level of assurance. One example of one that does would be the Mars Pathfinder Rover, which is a TRL 7 technology demonstration for future Mars micro-rovers of similar design.
TRL 8: Actual system completed and "flight qualified" through test and demonstration (ground or space), or "Gone gold"
By definition, all technologies being applied in actual systems go through TRL 8. At this point, you have completed a Theoretical First Unit (TFU), or otherwise gotten a product ready for primetime. Version 1.0, basically.
TRL 9: Actual system "flight proven" through successful mission operations, or "Kid tested, mother approved."
Once your product is in use, it's TRL 9 by definition. This TRL does not include any expansions, or upgrades, which have their own TRLs, as appropriate.