Backpack is a synonym of knapsack.
As nouns the difference between backpack and knapsack is that backpack is a knapsack, sometimes mounted on a light frame, but always supported by straps, worn on a person’s back for the purpose of carrying things, especially when hiking]], or on a student's back when [[carry|carrying books while knapsack is (us) a case of canvas or leather, for carrying items on the back.
As verbs the difference between backpack and knapsack is that backpack is to hike and camp overnight in backcountry with one's gear carried in a backpack while knapsack is to go hiking while burdened with a knapsack, usually overnight (or longer).
backpack
English
(wikipedia backpack)
Noun
(en noun)
A knapsack, sometimes mounted on a light frame, but always supported by straps, worn on a person’s back for the purpose of carrying things, especially when hiking]], or on a student's back when [[carry, carrying books.
A similarly placed item containing a parachute or other life-support equipment.
knapsack
English
Noun
(en noun)
(US) A case of canvas or leather, for carrying items on the back.
(cryptography) A set of values from which a subset is chosen.