Tutti is an Italian word literally meaning all or together. As a musical term, it is used in various ways. It may refer to an orchestral passage in which every member of the orchestra is playing at once. Alternatively, if an orchestrator wants a single first violin, second violin, cello, or double bass to play while the rest of that string player's section is silent for the duration of a passage, he writes solo in the part at this point and he writes tutti in the part at the point he wishes the rest of the section to resume playing.
In organ music, it indicates that the full organ should be used: all stops and all couplers. (This organ usage is sometimes instead indicated as .) To simplify this changing over, many organ consoles offer a toe stud or a piston to call the tutti: Pressing once begins the tutti, and pressing again reverts to the previous registration.