An affectionate term for the moderator of a mailing list, especially someone who has to wade into flamefests to keep things civil. The term usually applies equally to moderators of any gender, although some people occasionally use the term 'listdad'.
'Listmom' has been in use since 1993 or earlier; Google's memory includes one 1993 post on bit.listserv.politics saying that the listmom was trying to get back online, and a few messages in 1994 referring to 'listmom and listdad' as two specific people who had set policies on what topics were allowed on some BITnet lists. By 1995, the term was firmly established, with references to "our listmom's (sic) Dan and Mike" without further explanation.