Source : Webster's Revised Unabridged Dictionary (1913)

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.

   2. (Med.) Same as {Entasia}.

Source : WordNet®

entasis
     n : a slight convexity in the shaft of a column; compensates for
         the illusion of concavity that viewers experience when
         the sides are perfectly straight
     [also: {entases} (pl)]