Definition:Function

The process which is symbolised by an operator is called a function.

The operand(s) of the operator can be considered to be the input(s). The output of the function is whatever the operator is defined as doing with the operand(s).

A function is in fact another name for a mapping, but while the latter term is used in the general context of set theory and abstract algebra, the term function is generally reserved for mappings between sets of numbers.