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A knife (pl: knives; from Old Norse knifr 'knife, dirk') is a tool or weapon with a cutting edge or blade, usually attached to a handle or hilt. One of the earliest tools used by humanity, knives appeared at least 2.5 million years ago, as evidenced by the Oldowan tools. Originally made of wood, bone, and stone (such as flint and obsidian), over the centuries, in step with improvements in both metallurgy and manufacturing, knife blades have been made from copper, bronze, iron, steel, ceramic, and titanium. Most modern knives have either fixed or folding blades; blade patterns and styles vary by maker and country of origin.

Knives can serve various purposes. Hunters use a hunting knife, soldiers use the combat knife, scouts, campers, and hikers carry a pocketknife; there are kitchen knives for preparing foods (the chef's knife, the paring knife, bread knife, cleaver), table knife (butter knives and steak knives), weapons (daggers or switchblades), knives for throwing or juggling, and knives for religious ceremony or display (the kirpan).

Parts Main article: Knife making See also: Kitchen knife § Nomenclature Different parts of a knife indicated with numerals Characteristic parts of a knife A modern knife consists of:

the blade the handle the point – the end of the knife used for piercing the edge – the cutting surface of the knife extending from the point to the heel the grind – the cross section shape of the blade the spine – the thickest section of the blade; on a single-edged knife, the side opposite the edge; on a two-edged knife, more toward the middle the fuller – a groove added to make the blade lighter the ricasso – the flat section of the blade located at the junction of the blade and the knife's bolster or guard the guard – the barrier between the blade and the handle which prevents the hand from slipping forward onto the blade and protects the hand from the external forces that are usually applied to the blade during use the hilt or butt – the end of the handle used for blunt force the lanyard – a strap used to secure the knife to the wrist The blade edge can be plain or serrated, or a combination of both. Single-edged knives may have a reverse edge or false edge occupying a section of the spine. These edges are usually serrated and are used to further enhance function.

The handle, used to grip and manipulate the blade safely, may include a tang, a portion of the blade that extends into the handle. Knives are made with partial tangs (extending part way into the handle, known as "stick tangs") or full tangs (extending the full length of the handle, often visible on top and bottom). There is also the enterçado construction method present in antique knives from Brazil, such as the Sorocaban Knife, which consists in riveting a repurposed blade to the ricasso of a bladeless handle. The handle may include a bolster, a piece of heavy material (usually metal) situated at the front or rear of the handle. The bolster, as its name suggests, is used to mechanically strengthen the knife.