iron oxide

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of various oxides of iron, such as ferric oxide or ferrous oxide.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Chem.) Any of the oxides of iron, especially ferric oxide.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun chemistry The compound obtained by the reaction of oxygen on iron. Either iron(II) oxide (old term ferrous oxide, chemical formula FeO) or iron(III) oxide (old term ferric oxide, chemical formula Fe2O3). Generally known as rust when hydrated.