The genitive (cāsus patricus 'paternal case' in Latin) is the name for this second form ("-ae" for the first declension) and is easy to remember as the equivalent of a possessive or apostrophe-s case in English. That's not its complete role, though. In Latin, the genitive is the case of description. The use of one genitive noun limits the meaning of another noun, according to Richard Upsher Smith, Jr., in A Glossary of Terms in Grammar, Rhetoric, and Prosody for Readers of Greek and Latin: A Vade Mecum.
There are five declensions in Latin. The genitive ending is used in the dictionary because each of the five declensions has its own genitive form. The five genitive terminations are:
An example from each of the 5 declensions:
- puellae - the girl's (puella, -ae, f.)
- servī - the slave's (servus, -ī, m.)
- principis - the chief's (princeps, -ipis, m.)
- cornūs - the horn's (cornū, -ūs, n.)
- dieī - the day's (dies, -eī, m.)