The Collaborative International Dictionary of English v.0.48:

Ferrous \Fer"rous\, a. [Cf. F. ferreux. See Ferreous.] (Chem.)
   Pertaining to, or derived from, iron; -- especially used of
   compounds of iron in which the iron has its lower valence of
   two; as, ferrous sulphate.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ferrous
    adj 1: of or relating to or containing iron [syn: ferric,
           ferrous]