The second declension is a category of nouns in Latin and Greek with similar case formation.
[1] In Classical Latin, the short o of the nominative and accusative singular became u.
The latter class, i.e. the neuter nominative/accusative singular, usually ends with -um in Latin and -ον (-on) 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. For specifics on the second declension as it appears in Latin and Greek, see the appropriate sections in Latin declension and Ancient Greek nouns.
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