Someone who experiences little or no sexual attraction; an umbrella spanning a spectrum ('ace').
The load-bearing word here is attraction, not behavior. Asexuality is about who (or whether) you feel drawn to sexually — not about what you do, how high your libido runs, or whether you have sex. AVEN, the community that put the modern definition on the map, keeps it simple: an asexual person "does not experience sexual attraction." Everything else — desire, relationships, kink, arousal — sits on top of that, and varies wildly from one ace person to the next.
Which is exactly where the usual misreadings start. Asexual is not celibacy (that's a choice about behavior), not a hormone problem, not a symptom of trauma, and not "hasn't met the right person yet." It's also not the same as aromantic: the ace community separates sexual attraction from romantic attraction — the "split attraction model" — so plenty of ace people fall in love, date, and partner up; they just don't route it through sexual attraction.
In practice "ace" works as an umbrella, and the spectrum is where it gets specific. Gray-asexual is for people in the fuzzy zone — attraction that's rare, faint, or conditional. Demisexual describes only feeling sexual attraction after a strong emotional bond forms. Those terms weren't handed down by sexologists; they were coined on the AVEN forums in 2006 by users sorting out their own experience, and "ace spectrum" settled in as the catch-all around 2011. It's a vocabulary a community built for itself, which is the whole point.