sigmoidal

{ } a. [ Gr. &unr_;&unr_;&unr_;; &unr_;&unr_;&unr_; sigma + e'i^dos form, likeness: cf. F. sigmoïde. ] Curved in two directions, like the letter S, or the Greek ς. [ 1913 Webster ]

Sigmoid flexure (Anat.), the last curve of the colon before it terminates in the rectum. See Illust. under Digestive. --
Sigmoid valves. (Anat.) See Semilunar valves, under Semilunar. [ 1913 Webster ]