adj. un·sta·bler, un·sta·blest
a. Tending strongly to change: unstable weather.
b. Not constant; fluctuating: unstable vital signs.
a. Wavering or fickle: At first, he was unstable in his decisions.
b. Showing or marked by erratic or volatile emotions or behavior.
3. Not firmly placed; unsteady: an unstable ladder.
a. Decomposing readily.
b. Highly or violently reactive.
a. Decaying with relatively short lifetime. Used of subatomic particles.
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2020 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Indo-European & Semitic Roots Appendices
Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:
The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.