I’ve found this to be a language and programming-paradigm thing. One paradigm — OOP — refers to objects with member methods, which conceptually are how you send messages to those objects (this view is reflected in UML, for example).
Another paradigm — functional — may or may not involve classes of objects, but functions are the atomic unit of work.
In structured programming, you had sub-routines (notice that the prefix “sub” implies structure).
In imperative programming (which overlaps with structured quite a lot, but a slightly different way of looking at things), you have a more formulaic view of the world, and so ‘functions’ represent some operation (often mathematical).
All you have to do to not sound like a rube is to use the terminology used by the language reference for the language you’re using.