If you'd like to check it out for yourself, the definition is on page 47 of Spivak's Calculus. That is Chapter 3, "Functions".

Just to give you the definition that comes after the definition of function:

If f is a function, the domain of f is the set of all a for which there is some b such that (a,b) is in f. If a is in the domain of f, it follows from the definition of a function that there is, in fact, a unique number b such that (a,b) is in f. This unique b is denoted f(b).