unstable

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Tending strongly to change.
• adjective Not constant; fluctuating.
• adjective Wavering or fickle.
• adjective Showing or marked by erratic or volatile emotions or behavior.
• adjective Not firmly placed; unsteady.
• adjective Decomposing readily.
• adjective Highly or violently reactive.
• adjective Decaying with relatively short lifetime. Used of subatomic particles.
• adjective Radioactive.

from The Century Dictionary.

• To make no longer a stable or filthy abode.
• Not stable; not fixed.
• Not steady; inconstant; irresolute; wavering.

from the GNU version of the Collaborative International Dictionary of English.

• adjective Not stable; not firm, fixed, or constant; subject to change or overthrow.
• adjective See Stable equilibrium, under Stable.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective having a strong tendency to change
• adjective fluctuating; not constant
• adjective fickle
• adjective unpredictable
• adjective chemistry Readily decompasable
• adjective physics Radioactive, especially with a short half-life

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• adjective subject to change; variable
• adjective disposed to psychological variability
• adjective highly or violently reactive
• adjective lacking stability or fixity or firmness
• adjective affording no ease or reassurance
• adjective suffering from severe mental illness