Which term describes the return to the original speed in music?

The term that describes the return to the original speed in music is "tempi," which comes from the singular form "tempo." In musical terminology, tempo indicates the pace of the music, and when a performance indicates a return to a previous tempo, it is often referred to as bringing the music back to its original tempo.

In contrast, the other options have different meanings within the context of music. "Ritardando" refers to a gradual slowing down of the tempo, "ad libitum" indicates a performer can choose to vary the tempo or omit certain notes according to their interpretation, and "animato" suggests a lively or animated manner of playing but does not specifically denote a return to the original speed. Therefore, the concept of returning to the original speed is most accurately encapsulated by using the term tempo.