The Collaborative International Dictionary of English v.0.48:

bullsnake \bull"snake`\, bull snake \bull"
snake`\(b[oo^]l"sn[=a]k`), n.
   any of several large harmless rodent-eating North American
   burrowing snakes of the genus Pituophis. They include the
   gopher snake and pine snake.

   Syn: .
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

gopher snake
    n 1: large dark-blue nonvenomous snake that invades burrows;
         found in southern North America and Mexico [syn: indigo
         snake, gopher snake, Drymarchon corais]
    2: bull snake of western North America that invades rodent
       burrows [syn: gopher snake, Pituophis melanoleucus]