A dagger is a fighting knife with a very sharp point and usually two sharp edges, typically designed or capable of being used as a thrusting or stabbing weapon. Daggers have been used throughout human history for close combat confrontations, and many cultures have used adorned daggers in ritual and ceremonial contexts. The distinctive shape and historic usage of the dagger have made it iconic and symbolic. A dagger in the modern sense is a weapon designed for close-proximity combat or self-defense; due to its use in historic weapon assemblages, it has associations with assassination and murders. Double-edged knives, however, play different sorts of roles in different social contexts.

A wide variety of thrusting knives have been described as daggers, including knives that feature only a single cutting edge, such as the European rondel dagger or the Afghan pesh-kabz, or, in some instances, no cutting edge at all, such as the stiletto of the Renaissance. However, in the last hundred years or so, in most contexts, a dagger has certain definable characteristics, including a short blade with a sharply tapered point, a central spine or fuller, and usually two cutting edges sharpened the full length of the blade, or nearly so. Most daggers also feature a full crossguard to keep the hand from riding forwards onto the sharpened blade edges.

Daggers are primarily weapons, so knife legislation in many places restricts their manufacture, sale, possession, transport, or use.