The word for backpack, knapsack or rucksack in Italian is zaino (masculine, plural: zaini).
It refers to any bag with shoulder straps, including those carried by hikers, soldiers, and school children.
Non mi serve una valigia. Lo zaino è più che sufficiente.
I don’t need a suitcase. The backpack is more than enough.
According to Treccani, it is thought that the word may derive from the Lombardic term zaina meaning basket (cesto / canestro).
The diminutive zainetto literally means small backpack but it is often used to specifically refer to a child’s school backpack.
Non dimenticare lo zainetto quando vai a scuola, Beatrice!
Don’t forget your backpack when you go to school, Beatrice!
Because there isn’t an equivalent for the verb to backpack in Italian, you have to say viaggiare con lo zaino in spalla (to travel with your backpack on your shoulders).