unstable

adj. unsteady, insecure, shaky; changeable, unpredictable

Instability in systems is generally characterized by some of the outputs or internal states growing without bounds. Not all systems that are not stable are unstable; systems can also be marginally stable or exhibit limit cycle behavior.In control theory, a system is unstable if any of the roots of its characteristic equation has real part greater than zero. This is equivalent to any of the eigenvalues of the state matrix having real part greater than zero.In structural engineering, a structure can become unstable when excessive load is applied. Beyond a certain threshold, structural deflections magnify stresses, which in turn increases deflections. This can take the form of buckling or crippling. The general field of study is called structural stability.
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Unstable, adj 1. not firm or fixed in one place; likely to move. 2. capable of undergoing spontaneous change. [more]Unstable - Community and Resources

Adjective
1. lacking stability or fixity or firmness; "unstable political conditions"; "the tower proved to be unstable in the high wind"; "an unstable world economy"
(antonym) stable
(similar) coseismic, coseismal
(see-also) impermanent, temporary
2. highly or violently reactive; "sensitive and highly unstable compounds"
(similar) reactive
3. affording no ease or reassurance; "a precarious truce"
(synonym) precarious
(similar) uneasy
4. suffering from severe mental illness; "of unsound mind"
(synonym) mentally ill, unsound
(similar) insane
5. disposed to psychological variability; "his rather unstable religious convictions"
(similar) irresolute
6. subject to change; variable; "a fluid situation fraught with uncertainty"; "everything was unstable following the coup"
(synonym) fluid
(similar) changeable, changeful

(a.)
Not stable; not firm, fixed, or constant; subject to change or overthrow.