Eosin Y, also known as Eosin Yellow, is a water-soluble xanthene dye used in histology, particularly in the H&E (hematoxylin and eosin) stain, to differentiate connective tissue and cytoplasm. It is an organobromine compound and an organic sodium salt, specifically a tetrabromo derivative of fluorescein. It's a fluorescent red dye and can be found in both water-soluble and ethanol-soluble varieties.