As a healthy young woman with no psychiatric history is falling asleep one night, she briefly and vividly hears her name being called, lasting only a few seconds before it passes and does not recur once she is either fully asleep or awake again. What is this?

This is a hypnagogic hallucination: a brief, vivid sensory experience occurring specifically in the transitional period while falling asleep, not recurring once fully awake or fully asleep, exactly as described. It is not a hallucination occurring during ordinary wakefulness, since a hypnagogic hallucination differs from that purely by its timing, which is the single defining feature — this occurs specifically in the sleep transition, not during full wakefulness. It is not a vivid dream, since hypnagogic hallucinations differ from a vivid dream, which occurs during sleep itself rather than in the transition into or out of it. And it is not a pseudohallucination, since this is experienced with the full perceptual quality of a genuine hallucination, located in external space, rather than with the qualified, internally-located quality of a pseudohallucination.