Air hole flaw
From Stamps of the World
This is a term applied to an uncoloured area, which is usually circular, which appears accidentally on a coloured area of the stamp. It is caused by a bubble of air entering the plate during the stereotyping process. When the plate cools, the bubble bursts and this causes a slight depression in the raised surface of the plate. This depression results in an uninked area when the plate is used for the printing of the stamps.
