Thrombocythaemia, also known as essential thrombocythemia, is a rare blood disorder characterized by an excess of platelets in the blood. However, there are several other terms and synonyms for thrombocythaemia, including primary thrombocytosis, thrombocytosis, and megakaryocytic leukemia. Thrombocytosis may also refer to a secondary condition that arises due to an underlying medical condition, while megakaryocytic leukemia is a severe form of the disorder. Despite differences in nomenclature, all terms refer to an abnormal increase in platelet counts, which can increase the risk of clotting and other complications, such as bleeding disorders. Treatment options depend on the severity of the condition and may include medications, blood thinners, or chemotherapy.