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lurker: n.

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.