Webster’s Revised Unabridged Dictionary · 1913 Unstable Un·sta′ble (ŭn·stā′b'l), a. [Cf. Instable.] Not stable; not firm, fixed, or constant; subject to change or overthrow. — Un·sta′ble·ness, n. Chaucer. Unstable equilibrium. See Stable equilibrium, under Stable. ‹ Unsquire Unstack ›