A declension is a decline, like a downward slope or something that gets worse. As a grammar word, declension is the way a word's ending changes to reflect its function in a sentence.
A hill's steep declension makes it easy to roll down, and if you think there’s a declension in quality of movies these days, you might just skip the theater. In another sense, if you've learned German or Greek, you're familiar with declension, as when a noun changes its form slightly to show that it's the object of a sentence. In English, we use prepositions and word order instead. The Latin root means "to lower or bend from."