Person who has less than 5% of the average user's number of interventions in an online community (sometimes even 0 interventions), yet seems to know
everything about everyone. He's the one saving all of your Myspace photos in his computer without ever leaving a comment on them. After a week of lurking, he learns the ways of an online community
better than very old members of it.
In real life, a lurker is a person who "happens" to overhear the most intimate of conversations and witness some key moments without many people even noticing he's there.