It refers to the system of changing the form of a noun, pronoun, or adjective to indicate its grammatical function in a sentence. For example, in Latin, the word "rosa" (rose) can be declined to form different cases:
* rosa (nominative case - subject)
* rosae (genitive case - possession)
* rosae (dative case - indirect object)
* rosam (accusative case - direct object)
* rosa (ablative case - prepositional phrase)
Therefore, "declension" itself is a noun that describes this grammatical process.