Eosinophilia, or an elevated eosinophil (EOS) count, is a condition characterized by an eosinophil level in the blood that exceeds the normal reference range, as determined by a complete blood count. Specifically, it is clinically defined as an eosinophil count of 500 cells or more per microliter of blood. This elevation typically indicates an immune response within the body, often pointing to underlying conditions such as parasitic infections, allergic reactions, or certain types of cancer, where eosinophils play a crucial role in the immune defense.