A decrease, reduction, or decline in amount, number, or degree; can be used as a noun or with 'suru' as a verb.
人口の減少が地方都市の課題になっている。
Population decline has become a problem for regional cities.
支出の減少で今年の予算は余裕が出た。
With reduced expenditures, this year's budget has some leeway.
データはここ数年で急速な減少を示している。
The data show a rapid decline over the past few years.