The Tremol-No is a patented device that is added to an electric guitar equipped with a tremolo/vibrato unit. It allows guitarists to turn their tremolo unit "on" (so they can raise or lower the pitch of the note with the tremolo unit) or "off" (where the tremolo unit doesn't move at all; no lowering or raising of the note pitch via the tremolo system).

For a list of which Tremol-No's work with which tremelo's here.