Hypereosinophilia is a medical term used for a condition where an individual has an abnormally high concentration of eosinophil cells in their blood. Eosinophil cells are a type of white blood cell which play an important role in the body's immune response and can be elevated during allergic reactions or infections. Synonyms for hypereosinophilia include eosinophilia, hyper-eosinophilic syndrome, and HES. Other related terms include hypereosinophilic syndrome with organ involvement (HES-OI), which refers to the condition when eosinophil levels are elevated, and organ damage has occurred, and idiopathic hypereosinophilic syndrome (IHES), a term used for unexplained hypereosinophilia cases. These terms are commonly used in medical literature and research to describe and study hypereosinophilia.