Senza is used in sheet music, but it isn't an instruction all on its own: It's Italian for "without," and it must be combined with another word to show what it is you're supposed to play or sing without.
Senza pedale, for example, is a fairly common instruction in piano music, meaning "without pedal." (This refers to the sustain pedal, which can be used to allow notes to blend together.) Other instructions using senza include senza sordino, "without the mute," for violinists or trumpeters, and senza tempo, literally "without tempo" — meaning that there is no strict tempo for the passage, and performers should feel free to adjust or vary the tempo and rhythm to express whatever they want to express.