In Screenwriting, the Theme is the underlying "life principle" that flows beneath the storyline through its entirety.  In other words, it is a "subject line" that defines the essence of the movie while giving the audience something with which they can relate.

Usage

The one-line Theme is often used when pitching a screenplay to possible buyers, investors or representation. You might also find the Theme on various advertisements for the film (I.e. posters, newspaper, handbills, etc.)

Examples

Angels are among us                                                                                           (Angels in the Outfield, City of Angels, Michael, Dogma)

Crime doesn't pay.                                                                                          (Scarface, Donnie Brasco, Dog Day Afternoon, Once Upon a Time in America)

Money can't buy happiness.                                                                                 (Can't Buy Me Love, Pretty in Pink, Class, Godfather III)

Loss of innocence                                                                                                       (E.T., The Blue Lagoon, Lord of the Flies)