(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

the company declined to comment

(especially of the sun) move downwards

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

a gradual and continuous loss of strength, numbers, quality, or value

a serious decline in bird numbers

a civilization in decline