Medial: 1. Pertaining to the middle; in or toward the middle; nearer the middle of the body. Medial is as opposed to lateral. For example, the medial side of the knee is the side closest to the other knee whereas the lateral side of the knee is the outside of the knee.
2. Within a multilayered structure, the center layer. The medial layer of the aorta is the middle layer. For example, necrosis of the medial layer of the aorta may lead to aortic rupture.
The word medial comes from the Latin medialis, from medius, meaning the middle. Medial is the adjective corresponding to the noun median, as in the median in a highway.
For a more complete listing of terms used in medicine for spatial orientation, please see the entry to "Anatomic Orientation Terms".