filius

Filius, filii is second declension masculine, and it has two quirks worth pinning down. The genitive singular is written filii in most texts but contracts to fili in verse and in older prose, which is what happens to every -ius noun. And the VOCATIVE is fili, not filie: nouns in -ius take a bare -i when you address them, so Vergilius is addressed as Vergili and Gaius as Gai. That single -i is the only place where the second declension breaks its own rule twice over.

The feminine partner is filia, filiae, which does something else awkward: its dative and ablative plural is usually filiabus rather than filiis, precisely so that a sentence about sons and daughters together can keep them apart. Dea and dea's plural deabus do the same.

It gives filial, affiliate and filiation, and it survives whole into the Romance languages as French fils, Italian figlio, Spanish hijo and Portuguese filho.