a (mathematics) (computing) Describing an action which, when performed multiple times, has no further effect on its subject after the first time it is performed.
a (mathematics) Said of an element of an algebraic structure (such as a group or semigroup) with a binary operation: that when the element operates on itself, the result is equal to itself.
a (mathematics) Said of a binary operation: that all of the distinct elements it can operate on are idempotent (in the sense given just above).