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FeO for ferrous oxide, ( iron(II) oxide); Fe2O3 for ferric oxide, (iron(III) oxide) and
Fe3O4 for ferrous ferric oxide, (iron (II,III) oxide)
The name of the compound Fe3O4 is called iron (II,III) oxide. (Wikipedia)
iron (III) oxide
It stands for the charge on the iron ion. In Iron III oxide iron has a 3+ charge.