What does Rōmaeus mean in Latin?

Rōmaeus, a noun, means “Romaean, Byzantine”.

What is the genitive of Rōmaeus?

The genitive singular is Rōmaeī. Rōmaeus is masculine, second declension.

In classical Latin, [roːˈmae̯.ʊs]; in ecclesiastical (church) Latin, [roˈmɛː.us].

noun · masculinesecond declension

Rōmaeus m sg (genitive Rōmaeī)

Classical[roːˈmae̯.ʊs]Ecclesiastical[roˈmɛː.us]

post ClassicalRomaean, Byzantine
[…] et nuntius serenissimi domini Imperatoris Romæorum […]1872, Official Correspondence, page 28:

Calque of Ancient Greek Ῥωμαῖος (Rhōmaîos), using Rōma + -eus.

Elsewhere

Southern Gallo-Romance

Italo-Romance

Padanian

Northern Gallo-Romance

adjectivefirst/second-declension adjective

Rōmaeus²

Rōmaeus (feminine Rōmaea, neuter Rōmaeum)

Classical[roːˈmae̯.ʊs]Ecclesiastical[roˈmɛː.us]

singular
form	masculine	feminine	neuter
nominative	Rōmaeus	Rōmaea	Rōmaeum
genitive	Rōmaeī	Rōmaeae	Rōmaeī
dative	Rōmaeō	Rōmaeae	Rōmaeō
accusative	Rōmaeum	Rōmaeam	Rōmaeum
ablative	Rōmaeō	Rōmaeā	Rōmaeō
vocative	Rōmaee	Rōmaea	Rōmaeum

plural
form	masculine	feminine	neuter
nominative	Rōmaeī	Rōmaeae	Rōmaea
genitive	Rōmaeōrum	Rōmaeārum	Rōmaeōrum
dative	Rōmaeīs	Rōmaeīs	Rōmaeīs
accusative	Rōmaeōs	Rōmaeās	Rōmaea
ablative	Rōmaeīs	Rōmaeīs	Rōmaeīs
vocative	Rōmaeī	Rōmaeae	Rōmaea

Calque of Ancient Greek Ῥωμαῖος (Rhōmaîos), using Rōma + -eus.

What does Rōmaeus mean in Latin?: Rōmaeus, a noun, means “Romaean, Byzantine”.
What is the genitive of Rōmaeus?: The genitive singular is Rōmaeī. Rōmaeus is masculine, second declension.
How do you pronounce Rōmaeus?: In classical Latin, [roːˈmae̯.ʊs]; in ecclesiastical (church) Latin, [roˈmɛː.us].