WordNet sense 1 (a condition inferior to an earlier condition; a gradual falling off from a better state):

declination

WordNet sense 2 (change toward something smaller or lower):

diminution

WordNet sense 3 (a gradual decrease; as of stored charge or current):

decay

WordNet sense 4 (a downward slope or bend):

declension, declivity, descent, fall, downslope

WordNet sense 5 (refuse to accept):

refuse, reject, refuse, reject, turn down, pass up

WordNet sense 6 (go down in value):

correct, slump

WordNet sense 7 (grow smaller):

wane, go down

WordNet sense 8 (grow worse):

worsen