adjective un·sta·ble \ ˌən-ˈstā-bəl \
Updated on: 14 May 2018

Definition of unstable

: not stable : not firm or fixed : not constant: such as
a : not steady in action or movement : irregular
  • an unstable pulse
b : wavering in purpose or intent : vacillating
c : lacking steadiness : apt to move, sway, or fall
  • an unstable tower
d (1) : liable to change or alteration
  • an unstable economy
  • unstable weather
(2) : readily changing (as by decomposing) in chemical or physical composition or in biological activity
e : characterized by lack of emotional control




play \ˌən-ˈstā-b(ə-)lē\ adverb

Examples of unstable in a Sentence

  1. an unstable nuclear reactor core

  2. the minute we put the books down on the unstable desk, the whole stack went crashing to the floor

Recent Examples of unstable from the Web

These example sentences are selected automatically from various online news sources to reflect current usage of the word 'unstable.' Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. Send us feedback.

First Known Use of unstable

13th century

in the meaning defined above

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Synonym Discussion of unstable

inconstant, fickle, capricious, mercurial, unstable mean lacking firmness or steadiness (as in purpose or devotion). inconstant implies an incapacity for steadiness and an inherent tendency to change.
    • an inconstant friend
fickle suggests unreliability because of perverse changeability and incapacity for steadfastness.
    • performers discover how fickle fans can be
capricious suggests motivation by sudden whim or fancy and stresses unpredictability.
    • an utterly capricious critic
mercurial implies a rapid changeability in mood.
    • made anxious by her boss's mercurial temperament
unstable implies an incapacity for remaining in a fixed position or steady course and applies especially to a lack of emotional balance.
    • too unstable to hold a job

UNSTABLE Defined for English Language Learners


Definition of unstable for English Language Learners

  • : likely to change

  • : not emotionally or mentally healthy

  • : not held in a secure position : likely to move or fall

UNSTABLE Defined for Kids


adjective un·sta·ble \ ˌən-ˈstā-bəl \

Definition of unstable for Students

: not stable
  • an unstable boat

Medical Dictionary


adjective un·sta·ble \ -ˈstā-bəl \

medical Definition of unstable

: not stable: as
a : characterized by frequent or unpredictable changes
  • a patient in unstable condition
b : readily changing (as by decomposing) in chemical composition or biological activity
  • unstable compounds
c : characterized by lack of emotional control or stability

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