(typically of something regarded as good) become smaller, fewer, or less; decrease.
Example: the birth rate continued to decline
politely refuse (an invitation or offer).
Example: Caroline declined the coffee
(especially of the sun) move downwards.
Example: the sun began to creep round to the west and to decline
(in the grammar of Latin, Greek, and certain other languages) state the forms of (a noun, pronoun, or adjective) corresponding to case, number, and gender.
noun
a gradual and continuous loss of strength, numbers, quality, or value.