A knapsack is a bag you carry on your back, usually by slipping your arms through two straps. It's another word for backpack, though knapsack often suggests something simpler and more rugged: a canvas or leather bag that hikers, soldiers, or campers might use.

Originally, a knapsack was specifically a bag for carrying food and supplies on a journey. Soldiers have carried knapsacks for centuries, stuffing them with rations, extra clothing, and equipment they need in the field.

Today, people use knapsack and backpack almost interchangeably, though knapsack sounds a bit old-fashioned. You might read about a character in a novel setting off with a knapsack slung over one shoulder, or hear your grandfather talk about the knapsack he carried while hiking through the mountains as a young man. Modern backpacks might have laptop sleeves and water bottle pockets, but they serve the same basic purpose as those sturdy old knapsacks: carrying what you need while keeping your hands free.