Idempotent is concept borrowed from mathematics. It just means: f(f(x))=x assuming f is a function (binary operators are just functions (maps) in mathematics,

Idempotent is concept borrowed from mathematics. It just means: f(f(x))=x assuming f is a function (binary operators are just functions (maps) in mathematics, too, e.g. +:NxN->N (read: "plus" is a map from the Cartesian product of N by N to N))