Unintended means not planned, not meant to happen, or not what someone had in mind. When something is unintended, it occurs as a side effect or accident rather than on purpose.

Scientists sometimes make unintended discoveries while researching something completely different. Alexander Fleming was studying bacteria when he accidentally left a petri dish uncovered, and mold grew on it. The mold killed the surrounding bacteria, an unintended result that led to the discovery of penicillin, one of the most important medicines ever found.

Unintended consequences are results that nobody predicted or wanted. If a city builds a new highway to reduce traffic, but more people start driving because it's convenient, making traffic worse than before, that's an unintended consequence. When you're working on a group project and accidentally delete someone else's work while trying to fix a spelling error, that's an unintended outcome of your helpful intentions.

The word reminds us that our actions can have effects beyond what we plan. A joke might cause unintended hurt if it touches on something sensitive you didn't know about. Good intentions don't prevent unintended problems, which is why thinking ahead matters.