What is the name for the ionic compound FeO?

FeO is Iron(II)oxide, archaically , Ferrous oxide.

Do NOT confuse with Fe2O3, which is Iron(III)oxide ; Archaically Ferric oxide.

The 'iron' atom is in different oxidation states for these two compounds.

NB

'Fe' is the chemical symbol for 'Iron', and comes from Latin 'Ferrum' for iron.