<n0:ComplexDefinition xmlns:n0="http://ncicb.nci.nih.gov/xml/owl/EVS/ComplexProperties.xsd#"><n0:def-definition>A rare congenital abnormality of the heart characterized by the presence of three atria. The right or left atrium is divided into two parts by fibromuscular tissue or a membrane. It may be associated with other heart congenital abnormalities.</n0:def-definition><n0:def-source>NCI</n0:def-source></n0:ComplexDefinition>