Noun
decline [pl. declines]
decrease [pl. decreases]
reduction [pl. reductions]
downturn [pl. downturns]
depression [pl. depressions]
slump [pl. slumps]
downtrend [pl. downtrends]
falloff [pl. falloffs]
wane [pl. wanes]
dip [pl. dips] , {ping}
...
falling {down, away}
sinking {Foundry.}