Prone to change or fail; not stable or steady. The unstable weather made it difficult to plan outdoor activities.
Emotionally or mentally unpredictable or erratic. His unstable behavior worried his friends and family.
Not firmly fixed; likely to move or fall. The ladder was unstable, making it dangerous to climb.
Having atoms, nuclei, or molecular structures that are liable to decay or decompose; not in a stable energy state. Carbon-14 is an unstable isotope used in archaeological dating.