SikhA or scalp-lock is a long tuft of hair left on top or on the back of the shaven head of a male Brahmin Vaishnava sadhu. It signifies one-pointed ('ekanta') focus on one's spiritual goal, devotion to Brahman, mainly in the form of Vishnu, or Krishna.

External links - images

A head with sikha from Nevali Cori.

A boy with sikha, sculpture, Notre Dame, France.

A Hare Krishna monk with sikha.