“It is usual in mathematics—outside of mathematical logic—to use the word function imprecisely and to apply it to forms such as y2 + x. Because we shall later compute with expressions for functions, we need a distinction between functions and forms and a notation for expressing this distinction. This distinction and a notation for describing it, from which we deviate trivially is given by Church.”
John McCarthy et al.
The function is the simplest construct able to describe a behavior. Functions ...
