The wikipedia article for technology readiness levels lists the NASA and European Union definitions. I've put these into a table:

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)

There are slight differences in wording (this seems to allow TRLs to be used for non-space applications in the EU). Otherwise, are there any significant differences between the NASA and EU definitions of TRLs?

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    $\begingroup$ Yeah, I know the ESA is not quite the EU, but it seemed the most appropriate geographic tag. $\endgroup$
    – DrSheldon
    Aug 17 '21 at 4:53

That wikipedia page is out of date and a bit terse. Out of date: For example, NASA's TRL 6 and 7 currently both use "operational environment" in lieu of "relevant environment" (TRL 6) and "space environment" (TRL 7). This has been the case since 2013 (or earlier). Terse: There are always descriptions that go along with those definitions. The article omits the descriptions.

The concept of technology readiness has evolved a bit over time (e.g., the changes in NASA's TRL 6 and 7). As the concept has been applied elsewhere it has been tailored to those different areas. NASA itself has two different sets, on for hardware and another for software. Within the US federal government, the concept is used at the Departments of Defense, Energy, and Transportation. While the definitions and descriptions vary slightly, they remain quite similar to one another. It appears the same applies to the EU usage of the concept as well: tailored but no significant differences.

  • $\begingroup$ And it isn't like there aren't a bunch of arguments about exactly where any given technology actually sits on the terse or more verbose table - you have to convince the Technology Readiness Assessment committee folks that you crossed over whatever line you had to in order to move to the next phase... $\endgroup$
    – Jon Custer
    Aug 17 '21 at 17:27

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