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-.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.