A cleft chin is created when the left and right halves of the mandible fail to fuse completely during embryonic development.
It's also referred to as a "bum chin," which on first reading I assumed referred to a beggar/hobo/beach bum etc. Then I saw this comment:
My grandmother, who was French Mauritian, always told me that in Creole it was called a fes d'ange (spelling to be verified) which translated means angel's bum.Oh - that kind of "bum."
Photo gallery, via The Presurfer.