Which term refers to notes that are grouped in periods of three?

The correct answer is triplets, which specifically refers to a grouping of notes that are played or sung in a rhythm that divides a beat into three equal parts. This concept is fundamental in music theory, as it allows for a specific rhythmic feel that contrasts with the more common grouping of notes in twos (like eighth notes).

Triplets are indicated in sheet music with a bracket or a '3' above or below the notes, which signals to the musician that they should play three notes in the same duration that typically accommodates two notes. This can create a flowing, lilting effect in music that adds variety and complexity.

Understanding triplets is crucial for musicians as they play various genres that incorporate these rhythmic patterns, emphasizing their use in jazz, classical, and contemporary music. This knowledge enhances a musician's ability to interpret and perform various pieces accurately.