/ˈdôrsəm/
Noun, pl. dorsa
1. The back or dorsal region on an animal. (wiktionary.org)
2. The back of the body of a vertebrate or any analogous surface (as the upper or outer surface of an organ or appendage or part. (wordnetweb.princeton.edu)
3. The upper side of animals that typically run, fly, or swim in a horizontal position, and the back side of animals (like humans) that walk upright. In vertebrates the dorsum contains the backbone. (wikipedia.org)