unstable

The word “unstable” means likely to shift, collapse, or change suddenly; not firmly fixed. In plain terms, not steady — physically shaky, emotionally erratic, or chemically prone to breaking down.

“Unstable” is pronounced uhn-STAY-buhl (/ʌnˈsteɪ.bəl/ in IPA).

Here is “unstable” used in a sentence: “The ladder felt unstable, so he climbed down carefully.”

Words with a similar meaning to “unstable” include unsteady and wobbly.

The opposite of “unstable” — its antonyms — is stable.

“Unstable” is an adjective.

“Unstable” is a fairly common word in modern English.

“Unstable” has 3 distinct senses listed in this dictionary. The most common is: likely to shift, collapse, or change suddenly; not firmly fixed.

"Unstable" means likely to shift, collapse, or change suddenly and can apply to structures, situations, emotions, or substances. "Unsteady" is narrower and usually describes shaky physical movement or balance, as in unsteady hands or an unsteady walk.

Both suggest a lack of steadiness, but "unstable" means prone to shifting, collapsing, or breaking down, while "volatile" emphasizes rapid, unpredictable, and often explosive change. A market can be unstable over time and volatile from moment to moment.

When applied to a person, "unstable" usually means emotionally erratic or prone to sudden mood changes. This use can sound clinical or judgmental, so it is often softened in everyday speech to phrases like "going through a hard time."

In chemistry and physics, "unstable" describes a substance or particle prone to breaking down or decaying, sometimes radioactively. An unstable element or compound tends to react or transform readily rather than staying unchanged.