The Collaborative International Dictionary of English v.0.48:

nighthawk \nighthawk\ n.
   1. A person who likes to be active late at night; a night
      owl.

   Syn: night owl, nightbird.
        [WordNet 1.5]

   2. (Zool.) Any of several mainly nocturnal North American
      goatsuckers, especially Chordeiles minor, or the related
      European goatsucker Caprimulgus europaeus, also called
      the nightjar.

   Syn: bullbat, mosquito hawk.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

nighthawk
    n 1: a person who likes to be active late at night [syn: night
         owl, nighthawk, nightbird]
    2: mainly nocturnal North American goatsucker [syn: nighthawk,
       bullbat, mosquito hawk]