noun
: a compound bone that forms the posterior part of the skull and surrounds the foramen magnum, bearing the condyle or condyles for articulation with the atlas, in higher vertebrates being usually composed of four more or less completely united elements and in man much curved and of trapezoid outline, ending in front of the foramen magnum in the basilar process, and bearing on its outer surface behind the foramen magnum the two curved transverse superior and inferior nuchal lines besides a median crest and protuberance