(typically of something regarded as good) become smaller, fewer, or less; decrease.
the birth rate continued to decline
politely refuse (an invitation or offer).
Caroline declined the coffee
(especially of the sun) move downward.
We have designed the extension to capture the last scraps of sun as it declines behind the hill in the early evening.
(in the grammar of Latin, Greek, and certain other languages) state the forms of (a noun, pronoun, or adjective) corresponding to cases, number, and gender.
Lyly declined English nouns as if they were Latin.