What vocal technique involves alternating between two or more pitches?

The technique that involves alternating between two or more pitches is melisma. Melisma is a vocal flourish that stretches a single syllable over multiple notes, often resulting in elaborate and ornate vocal lines. This technique is commonly used in various musical styles, including classical, gospel, and pop. It allows singers to express emotion and artistry by embellishing melodies with intricate vocal runs.

Vibrato, while a popular vocal technique, refers to a slight variation in pitch achieved by rapid oscillation of the voice, not the alternation of pitches. Glissando involves sliding from one pitch to another, creating a smooth transition between notes rather than alternating between distinct pitches. Phrasing pertains to the way a musician shapes a sequence of notes and rests, influencing the musical expression but does not directly involve the act of alternating pitches. Hence, melisma is the correct answer in this context, as it specifically captures the essence of alternating between pitches in vocal performance.