Not op but they may mean the term-of-art where a pure function has no internal state and no side effects, so it always has the same outputs given the same input

Not op but they may mean the term-of-art where a pure function has no internal state and no side effects, so it always has the same outputs given the same inputs. Although it's not technically part of the definition of functional purity, I'd add that's it's nice to have a function whose behavior is defined for every input in the domain.