The Collaborative International Dictionary of English v.0.48:

backpack \back"pack\ n.
   a bag carried on the back, supported by straps looped over
   the shoulders.

   Syn: knapsack; rucksack; haversack. [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

backpack \back"pack\ v. i.
   to hike while carrying a backpack; -- often used in the form
   go backpacking; as, to backpack through the forest.
   [PJC]

WordNet (r) 3.0 (2006):

backpack
    n 1: a bag carried by a strap on your back or shoulder [syn:
         backpack, back pack, knapsack, packsack,
         rucksack, haversack]
    v 1: hike with a backpack; "Every summer they are backpacking in
         the Rockies" [syn: backpack, pack]