Front of House — often a term used to describe a PA system, providing amplified sound to the audience, as distinctly opposed to the 'monitor system' providing foldback to the musicians or performers.

See Public Address System

A system originally intended to amplify a human voice using microphones and loudspeakers in order to pass important information to the public over a large area, such as in transport hubs or sports stadia. The term now also applies to boosting the volume of instruments and vocals at musical concerts.