Source : Webster's Revised Unabridged Dictionary (1913)

Vanadious \Va*na"di*ous\, a. (Chem.)
   Pertaining to, or containing, vanadium; specifically,
   designating those compounds in which vanadium has a lower
   valence as contrasted with the {vanadic} compounds; as,
   vanadious acid. [Sometimes written also {vanadous}.]