In computer science, an invariant is a value that doesn't change.
Taken from Wikipedia:
Although computer programs are typically specified in terms of what they change, it's equally important to know or specify the invariants of a program. This is especially useful when reasoning about the program. The theory of optimizing compilers, the methodology of design by contract, and formal methods for determining program correctness, all pay close attention to invariants in computer programs.
Programmers often make use of assertions in their code to make invariants explicit. Some object oriented programming languages have a special syntax for specifying class invariants.