means A person who pays attention to what someone else is saying or to a sound, taking it in rather than just hearing it.

from Built from 'listen,' which goes back to Old English 'hlysnan,' meaning to pay attention or give ear — a cousin of 'hlystan' and tied to an old Germanic root for hearing. Add the '-er' that turns a verb into the one who does it, and you get the one who listens. That same ancient root, by the way, is thought to lie behind the now-archaic 'list' meaning 'to be pleased' or 'to listen,' a faint echo of when hearing and heeding were the same act.