A malignant neoplasm involving the endocardium. The disease is endocardium cancer (Monarch Disease Ontology term MONDO_0003687). Also known as: cancer of endocardium, malignant endocardial neoplasm, malignant endocardial tumor, malignant endocardial tumour, malignant endocardium neoplasm, malignant neoplasm of endocardium, malignant neoplasm of the endocardium, malignant tumor of endocardium.