Tiaras can be defined as an ornamental, often jeweled, crown-like semicircle worn on the head by women on formal occasions. Traditionally, the word "tiara" refers to a high crown, often with the shape of a cylinder narrowed at its top, made of fabric or leather, and richly ornamented. The kings and emperors of some ancient peoples in Mesopotamia used it. The Assyrians used to include a pair of bullhorns as a decoration and symbol of authority and a circle of short feathers surrounding the tiaras top.

The Persian tiara was more similar to a truncated cone, without the horns and feathers but more jewels, and a conic-shaped tip at its top. In the Roman Catholic Church, the Papal Tiara is a high cap surrounded by three crowns and bearing a globe surmounted by a cross-worn by the Pope during certain ceremonies, being the symbol of his authority.

In modern times, however, a tiara is generally a semi-circular band, often metal-made and decorated with jewels, which is worn as a form of adornment and not as a symbol of rank. It is worn by women around their head or on the forehead as a circlet. They are frequently used to "crown" the winners of beauty pageants and are also sometimes worn by the bride as part of a wedding outfit. The fictional superheroine Wonder Woman is usually depicted wearing a tiara; hers can be thrown as a weapon, as can that worn by the character known as Sailor Moon.