entasis

from WordNet (r) 3.0 (2006)

entasis
    n 1: a slight convexity in the shaft of a column; compensates
         for the illusion of concavity that viewers experience when
         the sides are perfectly straight

from The Collaborative International Dictionary of English v.0.48

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.
      [1913 Webster]

   2. (Med.) Same as {Entasia}.
      [1913 Webster]