‖n. [ NL. See Pleuro-, and Sigma. ] (Bot.) A genus of diatoms of elongated elliptical shape, but having the sides slightly curved in the form of a letter S. Pleurosigma angulatum has very fine striations, and is a favorite object for testing the high powers of microscopes. [ 1913 Webster ]
‖n.; pl.Sigmas [ L., from Gr. &unr_;&unr_;&unr_;, &unr_;&unr_;&unr_;. ] The Greek letter Σ, σ, or ς (English S, or s). It originally had the form of the English C. [ 1913 Webster ]