Filius

Filius is a Latin noun meaning “son.” It is masculine and belongs to the second declension. In classical Latin, filiuss denotes the male child or descendant within a family and appears in everyday phrases such as filius meus (“my son”) as well as religious usage like Filius Dei (“Son of God”). The word also features in legal and genealogical contexts within Latin texts.

Declension and forms follow the standard pattern of a second-declension masculine noun. Singular forms include nominative filius, genitive filii, dative filio, accusative filium, and ablative filio. Plural forms are nominative filii, genitive filiorum, dative filiīs, accusative filios, and ablative filiīs. In some uses the vocative may appear as filie or a variant, depending on the author, but the standard paradigm above covers most classical occurrences.

Etymology and cognates: Filius is from Proto-Italic and has cognates in other Romance languages, such as Italian figlio, French fils, Spanish hijo, and Romanian fiu. Its semantic core—son and male descendant—is reflected across these languages. In English, the root appears in several derivatives: filial, meaning relating to a son or family; filiation, the act or line of descent; and related terms used in expressions like filial piety and filial obligations.

In summary, filius is a basic Latin term for a son, with a well-attested decline and a family of derivative terms that have informed related vocabulary in English and other Romance languages.