The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

   Unstable equilibrium. See Stable equilibrium, under
      Stable.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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, unstable]
    4: suffering from severe mental illness; "of unsound mind" [syn:
       mentally ill, unsound, unstable]
    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, unstable]

Moby Thesaurus II by Grady Ward, 1.0:

197 Moby Thesaurus words for "unstable":
   adrift, afloat, alternating, ambiguous, amorphous, borderline,
   brittle, buoyant, canted, capricious, changeable, changeful,
   changing, choppy, corruptible, crumbling, dangerous, deciduous,
   decrepit, desultory, deviable, deviating, deviative, deviatory,
   dickey, different, disintegrating, disorderly, divaricate,
   divergent, diversified, diversiform, dizzy, doubtful, dubious,
   dying, eccentric, effervescent, elastic, ephemeral, erose, erratic,
   evanescent, fading, fast and loose, fickle, fitful, fleeting,
   flickering, flighty, flitting, fluctuant, fluctuating, fluid,
   fly-by-night, flying, fragile, frail, freakish, fugacious,
   fugitive, giddy, hazardous, heeling, ill-balanced, impermanent,
   impetuous, impulsive, inconsistent, inconstant, indecisive,
   indefinite, infirm, insecure, insubstantial, irregular, irresolute,
   irresponsible, jagged, jerky, leaning, listing, lopsided,
   lubricious, mazy, mercurial, mobile, momentary, moody, mortal,
   motley, moving, mutable, nonconformist, nondurable, nonpermanent,
   nonstandard, nonuniform, off-balance, overbalanced, passing,
   perilous, perishable, pluralistic, poor, poorish, precarious,
   protean, provisional, ragged, rambling, resilient, restless, risky,
   rocky, rootless, rotten, rotten at, rough, roving, scatterbrained,
   shaky, shapeless, short-lived, shuffling, slippery, spasmodic,
   spineless, sporadic, suspect, temperamental, temporal, temporary,
   tentative, tergiversating, ticklish, top-heavy, tottery, transient,
   transitive, transitory, treacherous, tricky, unaccountable,
   unbalanced, uncertain, uncontrolled, undecided, undependable,
   undisciplined, undurable, unenduring, unequable, unequal, uneven,
   unfaithworthy, unfirm, unfixed, unhealthy, unorthodox,
   unpredictable, unreliable, unrestrained, unsafe, unsettled,
   unsolid, unsound, unstable as water, unstaid, unsteadfast,
   unsteady, unsturdy, unsubstantial, unsure, unsystematic,
   untrustworthy, ununiform, vacillating, vagrant, variable,
   variegated, variform, various, varying, vicissitudinary,
   vicissitudinous, volatile, wandering, wanton, wavering, wavery,
   wavy, wayward, weak, whimsical, wishy-washy, wobbly