1 no object (typically of something regarded as good) become smaller, fewer, or less; decrease.
1.1 Diminish in strength or quality; deteriorate.
2 with object - Politely refuse (an invitation or offer)
3 no object (especially of the sun) move downwards.
3.1 archaic Bend down; droop.
4 with object (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.
1 A gradual and continuous loss of strength, numbers, quality, or value.
mass noun ‘a civilization in decline’
1.1 archaic The sun's gradual setting.
1.2 archaic A disease in which the bodily strength gradually fails, especially tuberculosis.
Late Middle English from Old French decliner, from Latin declinare ‘bend down, turn aside’, from de- ‘down’ + clinare ‘to bend’.
It was an offer too good to refuse, so Dennis merely declined
it. Somehow Don Corleone accepted that.