Buttock Cleavage Girl

Buttock cleavage is minor exposure of the buttocks and the gluteal cleft between them, often because of low-slung trousers. The crena is another formal term for the cleft between the buttocks. Expressive term is the ass crack.
When faced with indecency issues back in the 1930s, W.G. Cassidy explained in an essay titled Private Parts: A Judicial View that it may come under "other private parts" in described by the Australian Law, though indecency generally covers the genital area.