Unstable, a.
Etymology: [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.
Shades of meaning
Fernald has no entry of its own for unstable; he lists it under precarious.
He lists it without comment; the entries above explain its neighbors.