Entry 1 sense Webster, 1913 Unstable /ən-stāb'-əl/ · Un·sta·ble · IPA /ʌnˈsteɪ.bəl/ 01 a. Not stable; not firm, fixed, or constant; subject to change or overthrow. 1. Not stable; not firm, fixed, or constant; subject to change or overthrow. Phrases & compounds Unstable equilibrium — See Stable equilibrium, under Stable.