It's often used in chemistry to distinguish between compounds containing iron in its +2 oxidation state (ferrous) and those containing iron in its +3 oxidation state (ferric).

For example:

* Ferrous oxide (FeO) contains iron in the +2 oxidation state.

* Ferric oxide (Fe₂O₃) contains iron in the +3 oxidation state.

"Ferrous" can also be used in a broader sense to refer to anything related to iron, like "ferrous metals" (metals containing iron).