Entasis

In architecture, entasis is the application of a convex curve to a surface for aesthetic purposes. Its best-known use is in certain orders of classical columns that curve slightly as their diameter is decreased from the bottom upward.

Yale architectural historian Vincent Scully argues that entasis emphasizes the weight of the roof of a building by making the building columns appear to buckle under the pressure distributed among them. Danish architect Steen Eiler Rasmussen believed that the effect represented strength by imitating the swelling of a strained muscle, a theory that accords well with the etymology of the word, from the Greek meaning “to strain”.

text source: Wikipedia