Tacet is Latin for "it is silent". It is a musical term to indicate that an instrument or voice does not sound. In vocal polyphony and in orchestral scores, it usually indicates a long period of time, typically an entire movement. In more modern music such as jazz, tacet tends to mark considerably shorter breaks.
It was common for early symphonies to leave out the brass or percussion in certain movements, especially in slow (second) movements, and this is the instruction given in the parts for the player to wait until the end of the movement.
It is also commonly used in accompaniment music to indicate that the instrument does not play on a certain run through a portion of the music, i.e., "Tacet 1st time."
A unique usage of this term is in John Cage's 1952 composition 4′33″. A tacet is indicated for all three movements, for all instruments. The piece lasts a total of 4 minutes and 33 seconds, without a note being played.