function

This term is defined in ECSS-M-ST-10C Rev. 1 as
combination and interaction of a number of operations or processes, which together achieve a defined objective

This term is defined in ECSS-S-ST-00-01C as
intended effect of a product

This term is defined in ECSS-E-ST-10-06C as
“intended effect of a system, subsystem, product or part