The term that describes the optical trick of swelling columns at midpoint, which was used in the design of the Parthenon is entasis.
If you take a look at the ancient Greek columns, you will see that they are a bit wider in the middle - so the column is not completely straight, but rather swells up in the middle so as to "correct the visual illusion of concavity." It was often used by artists in the ancient Greek times.
