Final answer:
A tooth root attached to a jawbone by a periodontal ligament is a gomphosis, a type of immovable fibrous joint known as a peg-and-socket that secures teeth in the jaw.
Step-by-step explanation:
The type of joint where a tooth root is attached to a jawbone by a periodontal ligament is a gomphosis. A gomphosis, also known as a peg-and-socket joint, is a specialized fibrous joint that anchors the root of a tooth into its bony socket within the maxillary bone or mandible bone of the skull. The periodontal ligament consists of numerous short bands of dense connective tissue that spans between the bony walls of the socket and the root of the tooth. Owing to the immobility of a gomphosis, this type of joint is functionally classified as a synarthrosis, which is an immovable joint.