What is the musical technique exemplified by frequent alternation between two notes, like in a quarter note length?

The musical technique characterized by frequent alternation between two notes, typically in quick succession and often for the length of a quarter note, is known as a trill. A trill involves rapidly repeating the principal note and the note directly above it in the scale, creating a lively, shimmering effect in the music. This technique is commonly used in various musical genres to enhance expressiveness and add excitement to melodies.

The other terms refer to different musical techniques. An arpeggio involves playing the notes of a chord in succession rather than simultaneously, creating a flowing sound. A glissando is a continuous slide from one pitch to another, rather than the discrete alternation found in a trill. A mordent, on the other hand, indicates a brief ornamentation which involves the rapid alternation between the principal note and its immediate lower or upper neighbor, but it's usually very brief and not sustained over a longer duration like a quarter note.