Undesirable means not wanted or not good to have around. When something is undesirable, people prefer to avoid it or wish it weren't there.

An undesirable outcome is one you hoped wouldn't happen, like rain on the day of your outdoor birthday party. Undesirable behavior in class might include talking during a lesson or forgetting to do homework. Scientists work to eliminate undesirable side effects from medicines, like drowsiness or upset stomach.

The word often describes things that cause problems or make situations worse. Weeds are undesirable plants in a garden because they steal water and nutrients from vegetables and flowers. Rust is an undesirable effect of leaving metal tools outside in the rain.

When you're trying to solve a problem or improve something, identifying undesirable elements helps you figure out what needs to change. A chef eliminates undesirable flavors from a recipe. An engineer removes undesirable friction from a machine. The opposite of undesirable is desirable, meaning wanted or wished for.