The Collaborative International Dictionary of English v.0.48:

Eosin \E"o*sin\, n. [Gr. ? dawn.] (Chem.)
   A yellow or brownish red dyestuff obtained by the action of
   bromine on fluoresce["i]n, and named from the fine rose-red
   which it imparts to silk. It is also used for making a fine
   red ink. Its solution is fluorescent.
   [1913 Webster]

WordNet (r) 3.0 (2006):

eosin
    n 1: a red fluorescent dye resulting from the action of bromine
         on fluorescein; used in cosmetics and as a biological stain
         for studying cell structures [syn: eosin, bromeosin]