Entasis - Entasis was used by the Greeks while building monuments to correct optical illusions. Entasis is Greek for swelling or tension and relates to a convex curve incorporated into upright structures like columns and spires, according to Encyclopedia Britannica.
If a column does not have entasis it will not appear to be straight particularly in very large buildings. The Greeks began using entasis with their Doric columns.
The Greeks, in proportioning their Doric columns, arrived at an apparent system of entasis that is very different from the method the Roman employed with their columns. No part of the outline of the Doric column is parallel with its axis or center line. From the very bottom, the shaft slopes in towards the center, this slope increasing as it nears the neck of the column, in portions of arcs of circles of a varying though large radius.
The Romans, used a much more simple method for the entasis on their columns. For the most part - the bottom third of the column in non-tapered or straight and the tapering begins at for the following 2/3rds of the shaft.