Backpack

A backpack, also called knapsack, schoolbag, rucksack, pack, booksack, bookbag, haversack, packsack, or backsack, is in its simplest frameless form, a fabric sack carried on one's back and secured with two straps that go over the shoulders; but it can have an external or internal frame, and there are bodypacks.

Backpacks are commonly used by hikers and students, and are often preferred to handbags for carrying heavy loads or carrying any sort of equipment, because of the limited capacity to carry heavy weights for long periods of time with hands.

Large backpacks, used to carry loads over 10 kilograms (22 lb), as well as smaller sports backpacks (e.g. running, cycling, hiking, and hydration), usually offload the largest part (up to about 90%) of their weight onto padded hip belts, leaving the shoulder straps mainly for stabilizing the load. This improves the potential to carry heavy loads, as the hips are stronger than the shoulders, and also increases agility and balance, since the load rides nearer the wearer's own center of mass.

The word backpack was first recorded in the United States in 1914 (OED). Moneybag and packsack were used prior, and now occur mainly as regionalisms.

The word rucksack is a German loanword mainly used in the US, UK, and other Western countries by hikers and campers. In Middle High German ruck(e) means "back" (dorsum), which led to the Upper German word ruggsack. In modern German the word "der Rucksack" is commonly used. The name rucksack is cognate with the Danish rygsæk, Norwegian ryggsekk, Dutch rugzak, Afrikaans rugsak, Swedish ryggsäck, and Russian рюкзак (rjukzak).

The word knapsack was the usual name for a rucksack or backpack up until the middle of the 20th century.

Alternative names include haversack from the German Hafersack meaning "oat sack" (which more properly describes a small cloth bag on a strap worn over one shoulder and originally referred to the bag of oats carried as horse fodder), Kraxe (a German rucksack with a rigid framework), and bergen (a large load-carrying rucksack, from a design issued by the British Army during the Second World War).

Backpacks in general fall into one of four categories: frameless, external frame, internal frame, and bodypack. A pack frame, when present, serves to support the pack and distribute the weight of its contents across the body more appropriately, by transferring much of the weight to the hips and legs. Most of the weight is therefore taken off the shoulders, reducing the chance of injury from shoulder strap pressure (many backpacks equipped solely with shoulder straps can affect the posture of a person carrying more than 14 kg (30 lbs)), as well as being less restrictive of the upper body range of motion. Most backpacks are capable of being closed with either a buckle mechanism, a zipper, or a dry-bag type closure, though a few models use a drawstring fitted with a cord lock for the main compartment.