The Collaborative International Dictionary of English v.0.48:

Pocketknife \Pock"et*knife`\, n.; pl. -knives.
   A knife with one or more blades, which fold into the handle
   so as to admit of being carried in the pocket.
   [1913 Webster]

WordNet (r) 3.0 (2006):

pocketknife
    n 1: a knife with a blade that folds into the handle; suitable
         for carrying in the pocket [syn: pocketknife, pocket
         knife]