1. a combining form meaning "shining," "appearing, seeming," used in the formation of compound words: phenocryst.
2. a combining form used in the names of chemical compounds that contain phenol or the phenyl group, are related to aromatic compounds, or derive from benzene: phenobarbital.
Also, esp. before a vowel, phen- .
[ phaeno- phaino- shining, comb. form of phaínein to shine, appear; in chemical senses, used orig. with reference to products from the manufacture of illuminating gas ]