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

decline

WordNet sense 2 (a downward slope or bend):

declension, declivity, descent, fall, downslope

WordNet sense 3 ((astronomy) the angular distance of a celestial body north or south of the celestial equator; expressed in degrees; used with right ascension to specify positions on the celestial sphere):

DEC, celestial latitude

WordNet sense 4 (a polite refusal of an invitation):

regrets