One of the `silent majority' in an electronic forum; one who posts occasionally or not at all but is known to read the group's postings regularly. This term is not pejorative and indeed is casually used reflexively: "Oh, I'm just lurking." Often used in `the lurkers', the hypothetical audience for the group's flamage-emitting regulars. When a lurker speaks up for the first time, this is called `delurking'.
The creator of the popular science-fiction TV series "Babylon 5" has ties to SF fandom and the hacker culture. In that series, the use of the term `lurker' for a homeless or displaced person is a conscious reference to the jargon term.