1. a class of nouns or pronouns or adjectives in Indo-European languages having the same (or very similar) inflectional forms
e.g. the first declension in Latin
2. a downward slope or bend
Synonym: descentdeclivityfalldeclinedeclinationdownslope
3. process of changing to an inferior state
Synonym: deteriorationdecline in qualityworsening
4. the inflection of nouns and pronouns and adjectives in Indo-European languages