The term "knapsack" can refer to two different things:

1. A backpack, which is a fabric sack carried on ones back and secured with two straps that go over the shoulders. Backpacks can have an external frame and are used to carry loads over 10 kilograms (22 lb) .

2. The knapsack problem, which is a famous dynamic programming problem that falls in the optimization category. The problem is named after a scenario where someone is constrained by a fixed-size knapsack and must fill it with the most valuable items. Given a set of items, each with a weight and a value, the goal is to maximize the value in a knapsack while remaining within the weight constraint. The problem has been studied for more than a century and appears in real-world decision-making processes in a wide variety of fields, such as finding the least wasteful way to cut raw materials, selection of investments and portfolios, and generating keys for the Merkle–Hellman and other knapsack cryptosystems.