An "invariant" is a property or condition that remains unchanged under certain transformations or operations. In mathematics and computer science, invariants are often used to prove the correctness of algorithms or to describe the behavior of systems. For example, in a sorting algorithm, the invariant might be that the elements before a certain index are always sorted. Invariants can also be found in physics, where they describe quantities that do not change even when conditions vary. For instance, the laws of physics are often invariant under transformations like rotation or translation, meaning they hold true regardless of the observer's perspective or position in space.