WordNet sense 1 (the season when the leaves fall from the trees):

autumn

WordNet sense 2 (the time of day immediately following sunset):

dusk, gloaming, nightfall, twilight, gloam, crepuscle, crepuscule, evenfall

WordNet sense 3 (a downward slope or bend):

declension, declination, decline, declivity, descent, downslope

WordNet sense 4 (when a wrestler's shoulders are forced to the mat):

pin

WordNet sense 5 (a sudden decline in strength or number or importance):

downfall

WordNet sense 6 (a free and rapid descent by the force of gravity):

drop

WordNet sense 7 (a sudden sharp decrease in some quantity):

dip, free fall

WordNet sense 8 (fall from clouds):

precipitate, precipitate, come down

WordNet sense 9 (come under, be classified or included):

come

WordNet sense 10 (touch or seem as if touching visually or audibly):

shine, strike

WordNet sense 11 (come into the possession of):

accrue

WordNet sense 12 (fall to somebody by assignment or lot):

light

WordNet sense 13 (be inherited by):

devolve, pass, return

WordNet sense 14 (lose an upright position suddenly):

fall down

WordNet sense 15 (move downward and lower, but not necessarily all the way):

descend, go down

WordNet sense 16 (fall or flow in a certain way):

flow, hang

WordNet sense 17 (come as if by falling):

settle

WordNet sense 18 (decrease in size, extent, or range):

decrease, diminish, lessen, decrease

WordNet sense 19 (a sudden drop from an upright position):

spill, tumble

WordNet sense 20 (the act of surrendering (under agreed conditions)):

capitulation, surrender