A hat is a head covering which is worn for various reasons, including protection against weather conditions, ceremonial reasons such as university graduation, religious reasons, safety, or as a fashion accessory. Hats which incorporate mechanical features, such as visors, spikes, flaps, braces or beer holders shade into the broader category of headgear.

In the past, hats were an indicator of social status. In the military, hats may denote nationality, branch of service, rank or regiment. Police typically wear distinctive hats such as peaked caps or brimmed hats, such as those worn by the Royal Canadian Mounted Police. Some hats have a protective function. As examples, the hard hat protects construction workers' heads from injury by falling objects, a British police Custodian helmet protects the officer's head, a sun hat shades the face and shoulders from the sun, a cowboy hat protects against sun and rain and an ushanka fur hat with fold-down earflaps keeps the head and ears warm. Some hats are worn for ceremonial purposes, such as the mortarboard, which is worn (or carried) during university graduation ceremonies. Some hats are worn by members of a certain profession, such as the Toque worn by chefs, or the mitre worn by Christian bishops. Adherents of certain religions regularly wear hats, such as the turban worn by Sikhs, or the church hat that is worn as a headcovering by Christian women during prayer and worship.