rit. is one of the abbreviations that carries more than one meaning. Written out, it can be ritardando or ritenuto — 2 separate directions that happen to shorten to the same letters. The score itself decides which: the surrounding passage, the instrument, and what the music is doing at that moment.
What rit. can stand for
- ritardando Ritardando (rit.; also ritard.) directs a gradual slowing of the tempo. The longer abbreviation avoids the ambiguity of rit., which can also stand for ritenuto.
- ritenuto Ritenuto (most clearly riten.; also the ambiguous rit.) means 'held back': a slower tempo taken more immediately or held more uniformly than a gradual deceleration.