"A projection from the second cervical vertebra (axis) on which the first (atlas) can pivot", Early 19th century: from Greek odontoeidēs, from odous, odont- 'tooth' + eidos 'form'.[odontoid etymology, odontoid origin, 英语词源]
"A large fibroelastic ligament extending from the cervical vertebrae to the occiput in many mammals, serving as a support for the head; also called ligamentum nuchae", Mid 19th cent.; earliest use found in Richard Owen (1804–1892), comparative anatomist and palaeontologist.