Rack

A set X with a binary operation ▷ such that for all a in X the left translation L_a(x)=a ▷ x is a bijection of X, and the operation is left self-distributive: a ▷ (b ▷ c) = (a ▷ b) ▷ (a ▷ c). Racks capture algebraic self-action without requiring idempotence.