Unstable means not steady, secure, or reliable. When something is unstable, it's likely to change, fall, or fail without warning.

A stack of books piled too high becomes unstable: the slightest bump could send them tumbling. A wobbly table with one short leg is unstable, rocking back and forth whenever you set something on it. A rope bridge swaying in the wind feels unstable under your feet because it shifts unpredictably with each step.

The word also describes situations or conditions that keep changing. Unstable weather might switch from sunshine to thunderstorms within an hour. An unstable friendship shifts between closeness and distance, leaving you unsure where you stand. Scientists might describe an unstable chemical as one that reacts easily or breaks down quickly.

In science, unstable has a specific meaning: atoms or molecules that will naturally change into something else over time are called unstable. Radioactive materials are unstable because their atoms eventually break apart.

The opposite is stable: steady, secure, and dependable. When engineers design a bridge or building, they calculate carefully to make sure it's stable, not unstable, so it won't collapse. When you're learning to balance on one foot, you're working to stay stable instead of wobbling around in an unstable way.