n. a bag carried on the back or shoulder, supported by straps
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
n. a bag carried by a strap on your back or shoulder
Word Usage
"A backpack (also called rucksack, knapsack, packsack, pack, bookbag or Bergen) is, in its simplest form, a cloth sack carried on one's back and secured with two straps that go over the shoulders, but there can be exceptions."