Domain, in math, is defined as the set of all possible values that can be used as input values in a function. A simple mathematical function has a domain of all real numbers because there isn't a number that can be put into the function and not work.
An example in which the domain is not all real numbers is when a function results in an undefined number. For the function y= 3/(x-1), all real numbers work except for 1. When 1 is entered into the function, the end result is 3 divided by 0, which is an undefined number.