A kind of atrial septum defect arising from an enlarged foramen ovale, inadequate growth of the septum secundum, or excessive absorption of the septum primum. The phenotype is Human Phenotype Ontology identifier HP_0001684 (Secundum atrial septal defect). Also known as: Atrial septal defect, ostium secundum type, Ostium secundum atrial septal defect, Patent ostium secundum.