A knapsack is a bag with two straps that you wear over your shoulders, leaving your arms free. Don't forget your knapsack when you head out on that hiking trip!
You can also call a knapsack a "backpack" or a "rucksack." The word knapsack is thought to be the oldest of these terms, and while "backpack" is more often used today in the U.S., knapsack is more common in Canada. It comes from the German knappen, "to bite," and some experts believe that the name evolved from the fact that soldiers carried food in their knapsacks.