Source : Webster's Revised Unabridged Dictionary (1913)

Alligator wrench \Al"li*ga`tor wrench\ (Mech.)
   A kind of pipe wrench having a flaring jaw with teeth on one
   side.

Source : WordNet®

alligator wrench
     n : a wrench with a v-shaped jaw and serrations on one side
         (resembles the open jaws of an alligator)