: not stable: such as
a
: likely to change
unstable prices
unstable weather
b
: not emotionally or mentally healthy
She is emotionally/mentally unstable.
c
: not held in a secure position : likely to move or fall
an unstable tower/ladder
d
technical
: having a chemical structure or physical state that changes easily
an unstable nuclear reactor core
an unstable element
Cite this Entry
“Unstable.” Merriam-Webster.com Simple Definition, Merriam-Webster, https://www.merriam-webster.com/simple/unstable. Accessed 19 Jun. 2026.
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