dura mater

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. The tough fibrous membrane covering the brain and the spinal cord and lining the inner surface of the skull. It is the outermost of the three meninges that surround the brain and spinal cord.

Century Dictionary and Cyclopedia

1. The outermost membranous envelop or external meninx of the brain and spinal cord; a dense, tough, glistening fibrous membrane which lines the interior of the brain-case, but in the spinal column is separated from the periosteum lining the vertebræ by a space filled, with loose areolar tissue. In the skull it envelops the brain, but does not send down processes into the fissures. It forms, however, some main folds, as the vertical falcate sheet or falx cerebri between the hemispheres of the cerebrum, and the tentorium or horizontal sheet between the cerebrum and the cerebel-lum. Sundry venous channels between layers of the dura mater are the sinuses of the brain. The term dura mater is contrasted with pia mater, both these meninges being so named from an old fanciful notion that they were the “mothers,” or at least the nurses, of the contained parts.

Wiktionary

1. n. anatomy The tough and inflexible outermost of the three layers of the meninges

GNU Webster's 1913

1. (Anat.) The tough, fibrous membrane, which lines the cavity of the skull and spinal column, and surrounds the brain and spinal cord; -- frequently abbreviated to dura.

WordNet 3.0

1. n. the outermost (and toughest) of the 3 meninges