Is atrial septal defect considered a heart disease?

No: An ASD is a structural malformation of the heart that occurs when the normal sequence of events closing the atrial septum does not finish its work. While an untreated defect may result in a heart related problem (rhythm defect or congestive failure) the defect just makes you more vulnerable to a heart condition.

Yes: Even though "disease" may carry a connotation of "infection" in every day language, in medicine it means that there is some pathology or abnormality.

Yes: In medicine, there's "normal" and there's "disease". Since it isn't normal, by definition, it's a disease. We tend to think of disease as something you "catch" but congenital abnormalities are also diseases. Perhaps you're wondering if it's serious? Depends on size. Some are, some aren't.

Congenital defect: Asd's are congenital heart defects wherein blood is shunted from the left atrium (oxygenated) to the right atrium (deoxygenated) circulations. When this shunt is significant (shunt ratio greater than 1.8) or if there are symptoms of dyspnea or right sided pressure/volume overload (pulmonary hypertension), the asd's should be corrected either surgically or by catheterization techniques.

Yes and no: ASD is considered a congenital heart defect(present at birth). Failure to receive corrective treatment during childhood can lead to secondary heart conditions including rhythm disturbances, or heart dysfunction. It is more a matter of how you define the term.

Sort of: It is considered congenital heart disease as opposed to corornary artery disease.