The Collaborative International Dictionary of English v.0.48:

Phosphonic \Phos*phon"ic\ (f[o^]s*f[o^]n"[i^]k), a. [Phosphoric
   + sulphonic.] (Chem.)
   Pertaining to, or designating, certain derivatives of
   phosphorous acid containing a hydrocarbon radical with the
   phosphorus directly bonded to a carbon, and analogous to the
   sulphonic acids; as, methanephosphonic acid, CH3.PO.(OH)2.
   [1913 Webster + PJC]