de‧cline1 W2 [singular, uncountable]
There has been a decline in the size of families.
the decline of manufacturing
a rapid decline in unemployment
The island's population initially numbered 180, but there was a gradual decline until only 40 people were left.
the economic decline faced by many cities
widely held belief that educational standards are in decline (=falling)
fall/go etc into decline
The port fell into decline (=became less important and less busy) in the 1950s.