A sagittal crest is a ridge of bone running lengthwise along the midline of the top of the skull (at the sagittal suture) of many mammalian and reptilian skulls, among others.
The presence of this ridge of bone indicates that there are exceptionally strong jaw muscles. The sagittal crest serves primarily for attachment of the temporalis muscle, which is one of the main chewing muscles...
Sagittal crests are found in robust great apes, and some early hominids (Paranthropus). Prominent sagittal crests are found among male gorillas and orangutans, and do occur but only rarely in male chimpanzees such as Bili Apes.
The largest sagittal crest ever discovered in the human lineage belongs to the "Black Skull", Paranthropus aethiopicus, the earliest known robust hominid ancestor and the oldest robust australopithecine discovered to date. The prominence of the crest appears to have been an adaptation for the aethiopicus's heavy chewing, and the Black Skull's cheek teeth are correspondingly large. Smaller sagittal crests are also present on the skulls of other Paranthropines, including Paranthropus boisei and Paranthropus robustus.
Here is a picture of that skull: