verb

(typically of something regarded as good) become smaller, fewer, or less; decrease.

the birth rate continued to decline

decrease,reduce,get smaller,grow smaller,lessen,get less,diminish,wane,dwindle,contract,shrink,fall off,taper off,tail off,peter out,drop,fall,go down,sink,slump,plummet,plunge,nosedive,take a nosedive,take a header,go into a tailspin,crash

noun

a gradual and continuous loss of strength, numbers, quality, or value.

a serious decline in bird numbers

reduction,decrease,downturn,downswing,lowering,devaluation,depreciation,lessening,diminishing,diminution,slackening,waning,dwindling,fading,ebb,falling off,abatement,drop,slump,plunge,tumble,downtick,nosedive,crash,let-up,deterioration,degeneration,degradation,shrinkage,shrinking,withering,atrophy,weakening,enfeeblement,fall,failure,death,decay,decaying,decadence,devolution,declining,on the decline,crumbling,collapsing,atrophying,failing,disappearing,dying,moribund,past its prime,obsolescent,on its last legs,on the way out