Legitimate theatre is the term used to refer to plays that rely on the spoken word to convey the message oposed to musicals, reviews, dance, opera, or concerts.
An example of this is Romeo and Juliet. This is legitimate theater. Its counterpart, West Side Story which is the same basic story as Romeo & Juliet but with musical numbers throughout is not legitimate theater.
Sources:
Gillette, J. Michael. Theatrical Design and Production. 4th ed. Mountain View:
Mayfield, 1999