entasis

noun Architecture.

1. a slight convexity given to a column or tower, as to correct an optical illusion.

noun plural -ses (-siːz)

1. a slightly convex curve given to the shaft of a column, pier, or similar structure, to correct the illusion of concavity produced by a straight shaft
2. Also called: entasia (ɛnˈteɪzɪə) physiol an involuntary or spasmodic muscular contraction