mediastinum seminoma (Monarch Disease Ontology entry MONDO_0003826) is an extragonadal malignant germ cell tumor that arises from the mediastinum. It is characterized by the presence of uniform cells with clear or eosinophilic cytoplasm, round nucleus with one or more nucleoli, and distinct cellular borders. It usually arises from the anterior mediastinum. It may present with respiratory distress, chest pain, or superior vena cava syndrome or it may be asymptomatic, with the tumor detected on routine chest x-ray. The prognosis of mediastinal pure seminomas is favorable compared to the mediastinal non-seminomatous malignant germ cell tumors. Also known as: mediastinal seminoma, seminoma of mediastinum, seminoma of the mediastinum.