Urna: Tuft of hair between eyebrows

The mark on the forehead of a Buddha is called an urna and is described in the suttas as a tuft of white hair. It should not be confused with the third eye. On Buddha statues, the urna is sometimes represented as a single curl, as shown below (All images):



It is often depicted as a single dot as shown below (All images):




This is one of  the 32 Maha Purisa Lakkhana. The complete set of  marks are only found on Wheel-Turning Monarchs and Perfectly Enlightened Buddhas. They are the results of past wholesome actions and they predict a non-physical quality. The most detail about these marks can be found in the Lakkhana Sutta in the Digha Nikaya.

2.16 Monks, in whatever former life, former existence or dwelling-place the Tathagata, being born a human being, rejecting false speech, put away lies and became a truth-speaker, wedded to the truth, reliable, consistent, not deceiving the world, by performing that kamma, heaping it up, lavishly and abundantly, at the breaking-up of the body after death he was reborn in a happy state, in a heavenly world, where he was endowed beyond other devas in ten respects: in length of heavenly life, beauty, happiness, splendour, influence, and in heavenly sights, sounds, smells, tastes and contacts. Falling away from there and coming to be reborn here on earth, he acquired this mark of the Great Man:

(13) his body-hairs separate, one to each pore, and
(31) the hair between his brows white and soft like cotton-down

Being endowed with this mark, if he keeps to the household life, he will become a wheel-turning monarch … Conquering without stick or sword, but by justice, he rules over this earth as far as its ocean-boundaries, a land open, uninfected by brigands, free from jungle, powerful, prosperous, happy and free from perils. As a ruler, how does he benefit? He will be obeyed by Brahmin householders.

And if he goes forth into homelessness, he will become a fully-enlightened Buddha … As such, how does he benefit? He will be obeyed by monks.