Fen

(1) The plural of fan

Fen is the irregular plural of fan. (Man is to men as fan is to fen. But fans is okay while "mans" isn't. Go figure.) Also neofan/neofen.

The terms are used interchangeably, but there's a sense that fen is somewhat more fannish, being unique to fandom, and therefore especially useful when distinguishing between fans, meaning "members of fandom,"and fans, the mundane term meaning "fanatics" or "followers" or "blowers of hot air" (not that we don't have plenty of those in fandom, too).

The plural was popularized after Art Widner, Louis Russell Chauvenet and Norm Stanley, the only three members of MeCon in 1943, solemnly voted in its adoption.

from Fancyclopedia 2 ca. 1959
Plural of fan, by analogy with man/men; it came into wide use after a Maine conference solemnly voted its adoption. But the term is not universally accepted, and some dislike it.
from Fancyclopedia 1 ca. 1944
Alternative plural for "fans", which came into general use after the Mecon solemnly voted its adoption.

(2) An Apazine by James Kepner

Fen was the title of a fapazine published by James Kepner in the mid-1940s.
Issue Date Pages
2 December 1944 8

This is a Stub APAzine page. Please extend it by adding information about when and by whom it was published, what APAs it appeared in, its impact on fandom, or by adding scans or links to scans.

(3) The Fannish Educational Network

FEN was a Chicago-area organization headed by Dick Smith that sent out various fanzines at educational postal rates during the 1980s and '90s. It was revived to sponsor Operacon.