From Open Energy Information

Technology readiness levels (TRLs) describe the suitability of a product or technology for a particular task, based on U.S. Department of Energy definitions, seen in Table 4 from

Relative Level of Technology Development Technology Readiness Level TRL Definition
Systems Operation 9 Actual system operated over the full range of expected mission conditions
System Commissioning 8 Actual system completed and qualified through test and demonstration
7 Full-scale, similar (prototypical) system demonstrated in relevant environment
Technology Demonstration 6 Engineering/pilot-scale, similar (prototypical) system validation in relevant environment
Technology Development 5 Laboratory-scale, similar system validation in relevant environment
4 Component and/or system validation in laboratory environment
Research to Prove Feasibility 3 Analytical and experimental critical function and/or characteristic proof of concept
2 Technology concept and/or application formulated
Basic Technology Research 1 Basic principles observed and reported