A taboo, also spelled tabu, is a social group's ban, prohibition, or avoidance of something (usually an utterance or behavior) based on the group's sense that it is excessively repulsive, offensive, sacred, or allowed only for certain persons. Such prohibitions are present in virtually all societies. Taboos may be prohibited explicitly, for example within a legal system or religion, or implicitly, for example by social norms or conventions followed by a particular culture or organization.

Taboos are often meant to protect the individual, but there are other reasons for their development. An ecological or medical background is apparent in many, including some that are seen as religious or spiritual in origin. Taboos can help use a resource more efficiently, but when applied to only a subsection of the community they can also serve to suppress said subsection of the community. A taboo acknowledged by a particular group or tribe as part of their ways aids in the cohesion of the group, helps that particular group to stand out and maintain its identity in the face of others and therefore creates a feeling of "belonging".

The meaning of the word taboo has been somewhat expanded in the social sciences to strong prohibitions relating to any area of human activity or custom that is sacred or forbidden based on moral judgment, religious beliefs, or cultural norms. "Breaking a taboo" is usually considered objectionable by society in general, not merely a subset of a culture.