fäˈsfōnēəm noun
( -s )
Etymology: New Latin, from phosph- + -onium
: a univalent ion PH 4 + or radical PH 4 analogous to ammonium that is derived from phosphine and is known especially in the form of salts (as phosphonium iodide PH 4 I) and organic derivatives [as tetraphenyl-phosphonium iodide (C 6 H 5 ) 4 PI] — compare quaternary ammonium compound