The TRL is a rough measure of the maturity of a technology. It is a frustrating measure because it is not standardized and means different things in different contexts, and is not even clear within the same environment.
The NASA definition is not entirely useless. See also the EU one:
| TRL | NASA usage[1] | European Union[2] |
|---|---|---|
| 1 | Basic principles observed and reported | Basic principles observed |
| 2 | Technology concept and/or application formulated | Technology concept formulated |
| 3 | Analytical and experimental critical function and/or characteristic proof-of concept | Experimental proof of concept |
| 4 | Component and/or breadboard validation in laboratory environment | Technology validated in lab |
| 5 | Component and/or breadboard validation in relevant environment | Technology validated in relevant environment (industrially relevant environment in the case of key enabling technologies) |
| 6 | System/subsystem model or prototype demonstration in a relevant environment (ground or space) | Technology demonstrated in relevant environment (industrially relevant environment in the case of key enabling technologies) |
| 7 | System prototype demonstration in a space environment | System prototype demonstration in operational environment |
| 8 | Actual system completed and "flight qualified" through test and demonstration (ground or space) | System complete and qualified |
| 9 | Actual system "flight proven" through successful mission operations | Actual system proven in operational environment (competitive manufacturing in the case of key enabling technologies; or in space) |