sigmoid

Concise Oxford English Dictionary © 2008 Oxford University Press:
sigmoid /ˈsɪgmɔɪd/

▶adjective

• 1 curved like the uncial sigma (resembling the letter c); crescent-shaped.

• 2 S-shaped.

• 3 Anatomy of or denoting the curved part of the colon leading into the rectum.

– derivatives
sigmoidal adjective.

– origin C17: from Gk sigmoeidēs, from sigma (see sigma).