a reduction in the amount or quality of something
decline in: There has been a steady decline in public services over recent years.
a sharp/steep/dramatic decline: a sharp decline in share prices
economic/moral decline: an effort to halt the economic decline of the region
fall/go/slip into a decline: The industry has now fallen into a decline.
in decline: Agriculture is in decline in many Third World countries.
on the decline: Thankfully, this is a disease that is now on the decline.
Synonyms and related words