thirddeclension

The third declension is a major class of nouns in Latin grammar. It is the most diverse of the four noun declensions, including masculine, feminine, and neuter nouns with a wide variety of stems and nominative singular endings. A defining feature is the genitive singular ending -is, from which the rest of the forms are derived. Because of the variety of stems, the nominative singular can be any consonant or vowel ending, making the paradigm less regular than the earlier declensions.

In the standard paradigm, most third-declension nouns have genitive singular -is, dative singular -i, and accusative singular -em for masculine and feminine nouns. Neuter nouns in the same declension typically have nominative and accusative forms that are identical in the singular, and their accusative plural mirrors the nominative plural. The neuter plural endings are -a, with genitive plural often -orum or -um in many stems. The ablative singular is commonly -e, though some nouns show variation. In the plural, the masculine and feminine nominative and accusative forms usually end in -es, while neuter nouns end in -a. Genitive plural endings are -um for most third-declension nouns and -ium for i-stem nouns; dative and ablative plurals use -ibus.

A notable subgroup is the i-stem nouns, which behave like third-declension nouns but have -ium in the genitive plural and sometimes other distinct patterns in the genitive singular and other forms. The third declension also contains many irregulars and borrowed terms from Greek and other languages. Common examples include rex, urbs, miles, nomēn, and civis, illustrating the class’s breadth and centrality to Latin vocabulary.