If you are a monk that shaves all or some of your hair for religious reasons, that bald spot is called a tonsure. The act of creating that spot is also known as tonsure.
In Buddhism, Hinduism and Christianity, hair is often considered a vanity. Men entering a religious order choose a tonsure as a way of denouncing the vanity and worldly ways represented by hair. At one time, tonsure was required if you wanted to enter the clergy, though today it rarely is. The tonsure represents a commitment to a more austere and less proud lifestyle.