not stable; not firm or firmly fixed; unsteady.

liable to fall or sway.

unsteadfast; inconstant; wavering

unstable convictions.

marked by emotional instability

an unstable person.

irregular in movement

an unstable heartbeat.

Chemistry . noting compounds that readily decompose or change into other compounds.

noun

instability , precariousness , ricketiness , shakiness , unsteadiness , wobbliness , insecureness , insecurity , unsureness