olemus
ISO/IEC/IEEE 24765:
süsteemi või komponendi võimeid suurendava muutmise hõlpsus
= ease with which a system or component can be modified to increase its storage or functional capacity

ISO/IEC 2382:
programmikeele võime lubada uusi tarindeid
ja kasutada neid vastavalt oma senisele süntaksile
=
capability of a programming language to allow the specification of new language constructs and their use in the same syntactic manner as the standard language constructs

ülevaateid
https://en.wikipedia.org/wiki/Extensibility

https://docs.oracle.com/cd/E17904_01/doc.1111/e17363/chapter06.htm#FPCON258