Nullifier

A unique value derived from a secret, published when the secret is 'spent', that marks it as used without revealing which commitment it corresponds to. In privacy systems (mixers, shielded pools), commitments hide assets while nullifiers prevent double-spending them anonymously; the same idea enforces 'one anonymous action per secret' in private voting, proof-of-personhood, and airdrops. Nullifiers must be uniquely and deterministically derived yet unlinkable to outsiders — subtle to get right.