Second declension
The second declension is a category of nouns in Latin and Greek with similar case formation. In particular, these nouns are thematic, with an original o in most of their forms. In Classical Latin, the short o of the nominative and accusative singular became u.
Both Latin and Greek have two basic classes of second-declension nouns: masculine or feminine in one class, neuter in another. Most words of the former class have -us or -ος -os in the nominative singular, except for the r-stem nouns in Latin, and the "Attic" declension and contracted declension in Attic Greek. The latter class, i.e. the neuter nominative/accusative singular, usually ends with -um, in Latin and -ον, in Greek, matching the accusative of the former.
In Latin, the masculine words of the second declension that end with -us in the nominative case, are differently declined from the latter in the vocative case: such words end with -e.
