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What is entasis in the columns of the Parthenon?

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The Parthenon temple in Athens, Greece, is illuminated at night, showing its iconic Doric columns and partially ruined structure atop the Acropolis. — Parthenon Night view of the Parthenon, Athens.

In the architecture of the Parthenon, entasis refers to an imperceptible, delicate convexity applied to the columns as they taper in diameter toward the top.12

This technique is a deliberate architectural refinement intended to correct the optical illusion of hollowness or weakness that would otherwise occur with straight-sided tapering. By introducing this subtle curve, the architects ensured that the columns appear visually perfect to the human eye. This feature is part of a broader set of refinements in the temple, which also include the upward curvature of the base and the intentional thickening of the four corner columns to counteract the thinning effect caused by viewing them against the sky.321