# What is a non-expansive operator?

In the question Prove that projection operator is non-expansive, the author wants to prove that an operator is non-expansive. In the context of reinforcement learning, I heard that the $$\max$$ and $$\min$$ operators are non-expansive.

But what exactly is a non-expansive operator? What is the definition of an non-expansive operator? Why exactly exactly a max is non-expansive?