Legitimate theatre is live performance that relies almost entirely on diegetic elements, with actors performing through speech and natural movement. Traditionally, performances of such theatre were termed legitimate drama, while the abbreviation the legitimate refers to legitimate theatre or drama and legit is a noun referring both to such dramas and actors in these dramas. Legitimate theatre and dramas are contrasted with other types of stage performance such as musical theatre, farce, revue, melodrama, burlesque and vaudeville, as well as recorded performances on film and television.

Diegetic elements are internal, or related to the imaginary world the characters of the play are depicting. Musical accompaniment that is done for effect on the audience, but not supposed to be part of the experience of the characters in the play, is a non-diegetic element; dancing by characters who are actually portrayed as attending a dance is diegetic; while dancing for the benefit of the audience, as in a revue or musical, is non-diegetic. Opera, musicals, dance shows and concerts all depend on non-diegetic elements, while legitimate theater relies only on actors' spoken words and movements to relate a story. Legitimate theater typically relies on the talents of a small group of actors, or even a single actor, rather than a large, choreographed cast.

An example of the difference between legitimate theater and non-legitimate theater is the William Shakespeare play Romeo and Juliet compared with its modern adaptation for the musical West Side Story. In Shakespeare's play, the only dancing takes place at a ball where Romeo and Juliet meet. A sword fight is portrayed realistically, without singing or dancing. On the other hand, in West Side Story much of the plot is delivered in song, and a gang fight is portrayed through an elaborate dance.