Bonnet has been used as the name for a wide variety of headgear for both sexes—more often female—from the Middle Ages to the present. As with "hat" and "cap", it is impossible to generalize as to the styles for which the word has been used, but there is for both sexes a tendency to use the word for pop styles in soft material and lacking a brim, or at least one all the way round, rather than just at the front. Yet the term has also been used, for example, for steel helmets. This was from Scotland (in 1505), where the term has long been especially popular.

Headgear tied under the chin with a string was especially likely to be called a bonnet. Other features associated with bonnets as opposed to hats was that the forehead was not covered, and the back of the head often was. The outdoor headgear of female servants and workers was more likely to be called a bonnet. It was often worn outside over a thinner everyday head covering, which was worn at all times. In summary, hats were often stiffer, worn on the top of the head with the crown and brim roughly horizontal, while bonnets were pushed back, covering the back of the head, with any brim often approaching the vertical at the front. Other types of bonnet might otherwise be called "caps", for example the Scottish blue bonnet worn by working-class men and women, a kind of large floppy beret.

Bonnet derives from the same word in French, where it originally indicated a type of material. From the 18th century bonnet forms of headgear, previously mostly only worn by elite women in informal contexts at home, became adopted by high fashion, and until at least the late 19th century, bonnet was the dominant term used for female hats. In the 21st century, only a few kinds of headgear are still called bonnets, most commonly those worn by babies and Scottish soldiers. In addition, types of headgear called bonnets are worn by women as an outer Christian headcovering in some denominations such as the Amish, Mennonite and Brethren churches among the Anabaptist branch of Christianity, and with Conservative Quakers, mainly in the Americas.