Theater, one of the oldest and most popular forms of entertainment, in which actors perform live for an audience on a stage or in an other space designated for the performance. The space set aside for performances, either permanently or temporarily, is also known as a theater.

A prominent theater director, Peter Brook of Britain, has said that for theater to take place, an actor walks across an empty space while someone else is watching. In this empty space, called a stage, actors present themselves in a story about some aspect of human experience. The actors, the audience, and the space are three essentials of theater. The fourth is the performance, or the actors creative work in production. The performance is very often a play a tragedy, comedy, or musical but it need not be. Theater performances include vaudeville, puppet shows, mime, and other forms of entertainment.

Anthropologists and theater historians trace the origins of theater to myth and ritual found in dances and mimed performances by masked dancers during fertility rites and other ceremonies that marked important passages in life. Early societies acted out patterns of life, death, and rebirth associated with the welfare of village tribes. Imitation, costumes, masks, makeup, gesture, dance, music, and pantomime were some of the theatrical elements found in early rituals. At some unrecorded time, these ceremonies and rituals became formalized in dramatic festivals and spread west from Greece and east from India.