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.

The word we apply to the design principle is used by the Roman architect and engineer Vitruvius, and derives from the Greek word έντείνω (enteino), "to stretch or strain tight". Creating the illusion of greater strength or perception of height may have been an objective in the application of entasis.

Examples of this design principle may be found in cultures throughout the world, from ancient times to contemporary architecture. The first use of entasis is probably in the Later Temple of Aphaia at Aigina, in the 490s BC.

It may be observed among Classical period Greek column designs, for example, in the Doric order temples in Segesta, Selinus, Agrigento, and Paestum.

It was used less frequently in Hellenistic and Roman period architecture.^{[citation needed]} The Roman temples built during these periods were sometimes higher than those of the Greeks, with longer and thinner columns.

Chinese carpenters of the Song Dynasty followed designs in the AD 1103 Yingzao Fashi (Treatise on Architectural Methods or State Building Standards) that specified straight columns or those with an entasis on the upper third of the shaft.

Noted architects, such as the Renaissance master Andrea Palladio, also used entasis in the designs of their buildings.

Entasis was often a feature of Inca walls and doorways to counteract the optical illusion that would make the openings appear narrower in their middles.