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ad. Music Gradually slowing in tempo; retarding. Used chiefly as a direction.

ad. Gradually decelerating the tempo of a piece of music, especially at the end of the piece.

adj. Retarding; -- a direction for slower time; rallentado.

In music, becoming gradually slower; diminishing in speed: same as rallentando and (usually) ritenuto (but see the latter). Abbreviated rit. and ritard.

adj. gradually decreasing in tempo

"He followed this rhetorical question with the same two notes, ritardando, played four quaver chords, then a bar which disrupted one's expectations by the introduction of a rest and a pair of semiquavers, and very shortly broke into cascades of chorded and unchorded semiquavers that left Pelagia open-mouthed."

decreasing   

accelerando   

retarding    rallentado