entasis

thumb|upright|Diagram of a Corinthian order|Corinthian column showing a visible entasis bulge at "D"

In architecture, entasis is the application of a convex curve to a surface for aesthetic purposes, or increasing strength. Its best-known use is in certain orders of Classical columns that diminish in a very gentle curve, rather than in a straight line as they narrow going upward. The human eye would allegedly perceive that the middle of the column was diminishing in a concave curve halfway up the column, and entasis corrects this.