What is "vanadous"

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.]

a. Of, pertaining to or containing vanadium, especially in an oxidation number of 4.