Curl of Hair on Vishnu’s chest.
Bhrigu once wanted to find out who, Brahma, Vishnu or Shiva, is the most honorable. He went to visit them. Brahma and Shiva paid no attention to him, Vishnu was sleeping. Bhrigu kicked Vishnu in the upper body. He woke up and asked him if he was hurt. Bhrigu thus made Vishnu the victor. At the point where Bhrigu Vishnu stepped, the Shrivatsa curl of hair appeared.
See also Vishnu’s Insignia.