ENTASIS (from Gr. ἐντείνειν, to stretch a line or bend a bow), in architecture, the increment given to the column (q.v.), to correct the optical illusion which produces an apparent hollowness in an extended straight line. It was referred to by Vitruvius (iii. 3), and was first noticed in the columns of the Doric orders in Greek temples by Allason in 1814, and afterwards measured and verified by Penrose. It varies in different temples, and is not found in some: it is most pronounced in the temple of Jupiter Olympius, most delicate in the Erechtheum. The entasis is almost invariably introduced in the spires of English churches.