What defines a musical?

My girlfriend says something can only be considered a musical if there is a stage version of the play whereas I think that that musical only needs to have songs sung by the characters and that the songs have to move the plot forward. Does anyone have different ideas of what defines a musical?

What defines a musical?musicals

To answer what noone yet has. YOu are right and she is wrong. There are musicals based on movies and that have no other basis than their own concept. There need not be a straight play of the story. That is silly.

What defines a musical?opera songs opera theater

Watch any Fred Astaire, or Busby Berkeley movie, definition enough.
Musical theatre is a form of theatre combining music, songs, spoken dialogue and dance. The emotional content of the piece – humor, pathos, love, anger – as well as the story itself, is communicated through the words, music, movement and technical aspects of the entertainment as an integrated whole.

The three main components of a musical are the music, the lyrics, and the book. The book of a musical refers to the "play" or story of the show – in effect its spoken (not sung) lines; however, "book" can also refer to the dialogue and lyrics together, which are sometimes referred to (as in opera) as the libretto (Italian for “little book”). The music and lyrics together form the score of the musical. The interpretation of the musical by the creative team heavily influences the way that the musical is presented. The creative team includes a director, a musical director and usually a choreographer. A musicals production is also creatively characterized by technical aspects, such as set, costumes, stage properties, lighting, etc. that generally change from production to production (although some famous production aspects tend to be retained from the original production, for example, Bob Fosse's choregraphy in Chicago).

{http://en.wikipedia.org/wiki/Musical_the...
Boring, tedious, mind-numbing, soppy, annoying rubbish.
A musical must have at least one song for each character, either alone or as a duet.

The main characters' songs should be the "show stoppers."

The musical need not be portrayed entirely in song, there can be scenes of spoken dialogue.

Stage musicals are better, and most film musicals are adaptations of stage productions.
A musical in both theater and movie screen is a comb ination of song and dance.