The heart is split into four parts. Two collecting chambers at the top called Atria and two pumping chambers at the bottom called Ventricles. An Atrial septal defect (ASD) is a therefore a hole between the two top chambers - the Atria. There should not normally be a hole in this position as it allows blood to mix between the atria which should normally be separated. This is a reasonably common heart defect and often not to serious so don't be suprised if cardiologist does not hurry to to close the hole (with surgery or with a catheter in some cases).
Click here to see the British Heart Foundation leaflet on Atrial Septal Defect (ASD)
or click here to go to the British Heart Foundation website.