Ritardando (abbreviated rit.) is one of the most common performance markings in music, instructing the performer to gradually slow down. The effect is the temporal equivalent of a diminuendo — a gentle deceleration that creates a sense of arrival, conclusion, or breath.
The degree and rate of slowing are left to the performer's judgment, which is both the challenge and the expressive power of a ritardando. Too much slowing sounds heavy-handed; too little is imperceptible. In ensemble music, the conductor or lead player guides the group through the ritardando, ensuring everyone decelerates together. A tempo typically follows, restoring the original speed.
Glenn Gould was famous for taking enormous ritardandos at the ends of Bach pieces — his recording of the Goldberg Variations stretches the final bars to nearly twice the expected duration.