knapsack vs back pack

knapsack and back pack both are nouns.

Word	Noun	Adjective	Verb	Adverb
knapsack	Yes	No	No	No
back pack	Yes	No	No	No

As nouns, knapsack and back pack are synonyms defined as:

knapsack and back pack: a bag carried by a strap on your back or shoulder

Other synonyms of knapsack include backpack, haversack, packsack, rucksack.

knapsack (noun)	back pack (noun)
a bag carried by a strap on your back or shoulder	a bag carried by a strap on your back or shoulder