A monologue (or monolog) is a speech by one person directly addressing an audience. An opening monologue is often a segment of stand up comedy, such as at the beginning of The Tonight Show.
In a monologue, the actor need not be alone, however, none of the supporting cast (in theatre or film) speaks. When the actor is alone, perhaps thinking out loud, this is a soliloquy, not a monologue.
There are two basic types of monologues:
Exterior monologue: This is where the actor speaks to another person who is not in the performance space or to the audience.
Interior monologue: This is where the actor speaks as if to himself or herself. It is introspective and reveals the inner motives to the audience. This is also a common device in stream of consciousness writings.
The monologue is an artform in and of itself, but it has always been part of larger, dialogue-based, plays for its provision of another angle in relating to the audience and getting inside a character's inner life.
Famous monologists include Jack Parr, Bill Cosby, Lord Buckley, Spalding Gray, Mike Albo , Eric Bogosian, Garrison Keillor, Greg Walloch, and John Leguizamo.