In mathematical invariant theory, an evectant is a contravariant constructed from an invariant by acting on it with a differential operator called an evector. Evectants and evectors were introduced by Sylvester (1854, p.95).

• Sylvester, James Joseph (1853), "On the calculus of forms, otherwise the theory of invariants", The Cambridge and Dublin Mathematical Journal, 8: 257–269
• Sylvester, James Joseph (1854), "On the calculus of forms, otherwise the theory of invariants", The Cambridge and Dublin Mathematical Journal, 9: 85–103