What technique involves moving quickly and repeatedly between two notes that are a step apart?

The technique that involves moving quickly and repeatedly between two notes that are a step apart is known as a trill. In music, a trill typically involves the rapid alternation between a principal note and a note that is either a step above or below it. This creates a shimmering, decorative effect that adds musical embellishment and expressiveness to a piece.

Trills are often indicated in sheet music with the abbreviation "tr" and can occur in various styles and tempos, depending on the intention of the composer and the interpretation of the performer. The technique is commonly used in various musical genres, from classical to contemporary, to enhance melodic lines.

In contrast, portamento refers to a smooth, gliding transition from one note to another, while glissando involves sliding between pitches rather than jumping between specific notes. Vibrato, on the other hand, is a technique used to vary the pitch of a note slightly, adding warmth and richness to the sound but not involving repetition between two distinct pitches as in a trill.