The Collaborative International Dictionary of English v.0.48:

Chipmunk \Chip"munk`\, n. [Indian name.] (Zool.)
   A squirrel-like animal of the genus Tamias, sometimes
   called the striped squirrel, chipping squirrel, ground
   squirrel, hackee. The common species of the United States
   is the Tamias striatus. [Written also chipmonk,
   chipmuck, and chipmuk.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

chipmunk
    n 1: a burrowing ground squirrel of western America and Asia;
         has cheek pouches and a light and dark stripe running down
         the body