Which term indicates that notes should be played smoothly?

The term that indicates notes should be played smoothly is "slur." In musical notation, a slur is a curved line placed above or below a series of notes, which instructs the musician to connect them in a smooth, legato manner without re-articulating each note. This technique contrasts with staccato playing, where notes are played sharply and detached from each other.

The other terms listed serve different functions within musical performance. For instance, "spiritoso" suggests a lively or spirited performance, but it does not specifically dictate the smoothness of the notes. "Pretissimo" indicates a tempo that is extremely fast, which influences the speed of the performance rather than the articulation of the notes. Lastly, "tranquilo" relates to a calm or tranquil playing style, but again does not directly instruct on how to connect notes. Thus, while all terms evoke certain expressive qualities in music, only "slur" specifically directs the smooth articulation of notes.