• (n.) The hypothetical radical C3H5O, regarded as the essential residue of propionic acid and certain related compounds.
Pro'pi·o·nyl noun (Chemistry) The hypothetical radical C 3 H 5 O, regarded as the essential residue of propionic acid and certain related compounds. 
Type: Term Pronunciation: prō′pē-ō-nil Definitions: 1. the acyl radical of propionic acid. Synonyms: propanoyl