The Collaborative International Dictionary of English v.0.48:

nightcrawler \nightcrawler\ n.
   A terrestrial worm that burrows into and helps aerate soil;
   an earthworm. It often surfaces when the ground is cool or
   wet, and is used as bait by anglers. The term is used mostly
   in the northern and western U. S.

   Syn: earthworm, angleworm, fishworm, fishing worm, wiggler,
        nightwalker, dew worm, red worm.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

nightcrawler
    n 1: terrestrial worm that burrows into and helps aerate soil;
         often surfaces when the ground is cool or wet; used as bait
         by anglers [syn: earthworm, angleworm, fishworm,
         fishing worm, wiggler, nightwalker, nightcrawler,
         crawler, dew worm, red worm]