Which term refers to a gradual increase in tempo?

The term that refers to a gradual increase in tempo is "accelerando." This direction indicates that the performer should gradually play faster, creating a sense of urgency or excitement in the music.

In contrast, ritardando denotes a gradual slowing down of the tempo, while adagio and vivace refer to specific tempos themselves rather than descriptions of tempo changes. Adagio is a slow tempo, and vivace is fast and lively. Therefore, understanding the context and definitions of these terms helps to clarify why "accelerando" is the correct choice for indicating an increase in tempo.