TRL 1 Basic principles observed and reported.
TRL 2 Technology concept and/or application formulated.
TRL 3 Analytical and experimental critical function and/or characteristic proof-of-concept.
TRL 4 Technology basic validation in a laboratory environment.
TRL 5 Technology basic validation in a relevant environment.
TRL 6 Technology model or prototype demonstration in a relevant environment.
TRL 7 Technology prototype demonstration in an operational environment.
TRL 8 Actual Technology completed and qualified through test and demonstration.
TRL 9 Actual Technology qualified through successful mission operations.

NASA developed Technology Readiness Levels (TRLs) in the early 70’s as a means of assessing whether emerging technology was suitable for space exploration. By the 1990’s it was in use across many US Government agencies, including the Department of Defense (DOD) and Department of Energy (DOE). TRLs are now in common use in the UK.