Not-incompatible use
This is not a legal definition. It follows from Article 5(1)(b) of the GDPR (and Recital 50 of the GDPR).
Use for archiving in the public interest, scientific or historical research, or statistics, which the GDPR treats in advance as lawful and not incompatible with the original purpose. This is one of the four exceptions to purpose limitation. There is debate whether not-incompatible use also "rides along" on the original basis (like compatible use) or is simply permitted. That difference matters where there was no valid Article 6 ground for the original use: compatible use is then impossible, but not-incompatible use may still be allowed. The European Commission has proposed replacing "not incompatible" with "compatible".
Questions about what this term means for your organisation? Contact Mr. Dr. Vlieger.