A pilonidal cyst is a pocket of hair, skin and oils, that can be above your buttocks or on either side of the buttock cleft. A pilonidal cyst looks like a large pimple or bump.
Sometimes, a pilonidal cyst can get infected. When this happens, it may fill with pus (a thick white, yellow or green liquid made of dead skin cells and germs, like bacteria). This may be painful.