[ noun ] a slight convexity in the shaft of a column; compensates for the illusion of concavity that viewers experience when the sides are perfectly straight <noun.shape>
Entasis \En"ta*sis\, n. [NL., from Gr. ? a stretching; fr. ?; ? in + ? to extend.] 1. (Arch.) A slight convex swelling of the shaft of a column.