e·o·sin 
(ēə-sən)
(ēə-sən)n.
1. A yellowish-red crystalline powder, C20H8 Br4O5, used in textile dyeing and ink manufacturing, as a biological stain, and in coloring gasoline.
2. Any of a group of red fluorescent bromine derivatives of fluorescein, or their sodium or potassium salts, used in ink manufacturing, textile dyeing, and in biology to stain cells.
[Greek ēōs, dawn (from its color); see aus- in the Appendix of Indo-European roots + -IN.]