Off Broadway theater is an umbrella term for a defined set of plays, musicals or revues performed in New York City. Originally referring to the location of a venue and its productions on a street intersecting Broadway in Manhattan's Theatre District, the hub of the theater industry in the United States, the term later became defined by the League of Off-Broadway Theatres and Producers as a professional venue in New York City with a seating capacity between 99 and 500, or a specific production that appears in such a venue, and which adheres to related trade union and other contracts.

Previously, regardless of the size of the venue, a theatre could not be considered Off Broadway if it were within the "Broadway Box" (the traditional Broadway Theater District). The contractual definition changed this to encompass theaters meeting the standard, with a higher minimum salary requirement for Actors' Equity performers than for Off Broadway theaters outside the box.

A number of Off Broadway shows have had subsequent runs on Broadway. These have included the musicals A Chorus Line, Rock of Ages, Godspell, Avenue Q, Rent, In The Heights, Spring Awakening, Hair, Grey Gardens, Little Shop of Horrors, and Sunday in the Park with George, and the plays Doubt, I Am My Own Wife, Bridge & Tunnel and Coastal Disturbances. Other productions, such as Stomp, Blue Man Group, Altar Boyz, Perfect Crime and Naked Boys Singing have run for several years Off Broadway. The Fantasticks, the longest-running musical in theatre history, spent its original 42-year run Off Broadway.