Source : Webster's Revised Unabridged Dictionary (1913)

Unstable \Un*sta"ble\, 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}.

Source : WordNet®

unstable
     adj 1: lacking stability or fixity or firmness; "unstable political
            conditions"; "the tower proved to be unstable in the
            high wind"; "an unstable world economy" [ant: {stable}]
     2: highly or violently reactive; "sensitive and highly unstable
        compounds"
     3: affording no ease or reassurance; "a precarious truce" [syn:
         {precarious}]
     4: suffering from severe mental illness; "of unsound mind"
        [syn: {mentally ill}, {unsound}]
     5: disposed to psychological variability; "his rather unstable
        religious convictions"
     6: subject to change; variable; "a fluid situation fraught with
        uncertainty"; "everything was unstable following the coup"
        [syn: {fluid}]