Functions can map elements from the domain to the codomain in many ways. A function may “hit” every element in the codomain, or it may “miss” some. It may assign more than one x to a y or it may assign exactly one x to each y. We will understand our function better if we know which of these things it does. We will present precise formulations of these terms in this chapter, and we’ll reinforce the definitions with examples, nonexamples, exercises and problems for practice.