Ferrous is an adjective coming from the Latin ferrum (iron). One uses this word to indicate the presence of iron, mostly in metallurgy or related industries. There are also non-ferrous metals, which are metals that have no iron in them at all. Ferrous metals, as opposed to non-ferrous metals, can rust. Ferrous can also refer to iron in its +2 oxidation state.
"Ferrous" is also the name of a song by Peter Gabriel.