The Collaborative International Dictionary of English v.0.48:

Monkfish \Monk"fish\, n. (Zool.)
   (a) The angel fish (Squatina).
   (b) The angler (Lophius), esp. the goosefishes Lophius
       Americanus in America and Lophius piscatorius in
       Europe, used for food. --[MW10]
       [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

Lophius Americanus
    n 1: fishes having large mouths with a wormlike filament
         attached for luring prey [syn: goosefish, angler,
         anglerfish, angler fish, monkfish, lotte,
         allmouth, Lophius Americanus]