The European Commission has established the definitions of the levels as follows:
Research
TRL 1: Basic principles observed
This phase is characterised by fundamental research. The basic principles of the technology have been observed and there are assumptions about the working principles of this technology. However, no experimental evidence for this is yet available.
TRL 2: Technology concept formulated
The technological concept and the possible field(s) of application have been formulated.
TRL 3: Experimental proof of concept
The first laboratory tests or analytical studies have been completed with a "proof of concept" as a result.
Technical development
TRL 4: Technology validated in lab
The proof of concept has been validated in a laboratory environment, often using raw (low-fidelity) small-scale prototypes: basic components are integrated to evaluate how they work together.
TRL 5: Technology validated in relevant environment
The technology has been tested and validated in a relevant environment. Functional and refined (high-fidelity) prototypes are often used for this.
TRL 6: Technology demonstrated in relevant environment
The operation of the technology has been demonstrated in a relevant environment. The prototype's performance has not yet been optimised for the operational environment.
Demonstration
TRL 7: System prototype demonstration in operational environment
The technology is integrated into the final operational environment. The prototype is (near) an actual operating system. The focus is now on improving production design, certification, et cetera.
TRL 8: System complete and qualified
The technology is performing properly and the latest production problems have been solved.
Deployment
TRL 9: Actual system proven in operational environment
The technology is technically and commercially ready. The next steps are production and market introduction.