A feature on some mixing consoles that allows you to mute all channels unless they have the solo button depressed. No channels will be muted if no solo buttons are depressed.

NOTE: this is a dangerous switch because the solo button usually doubles as the cue button and if this is on something nasty will happen during the show when you try to cue something up in your cans. Usually the buttons have some kind of idiot proofing on them so you won't accidentally press them (gaurds, being placed alone with nothing on either side, etc.)