back pack vs knapsack

back pack and knapsack both are nouns.

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

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

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

Other synonyms of back pack include backpack, haversack, packsack, rucksack.

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