catastrophic, considerable, dramatic, drastic, large, major, marked, massive, serious, severe, significant, substantial
precipitous, sharp, steep
rapid, sudden
gentle, modest, slight
gradual, slow
continuing, progressive, steady, sustained
a steady decline in manufacturing

general, long-term, overall
absolute, inevitable, inexorable, irreversible, terminal
an industry in terminal decline

economic, industrial, moral, political, urban
the moral decline of the nation

mental, physical
population, price
national
seasonal
age-related

experience, fall into, go into, suffer
The cloth trade went into gradual decline.

cause, lead to
The increased price of gold led to the decline of his business.

arrest, halt, stem, stop
We must halt this decline in standards.

slow
prevent
reverse
offset
accelerate, hasten
see, witness
We have seen a sharp decline in educational standards over recent years.

lament
They lament the decline of old-fashioned communities.

occur
Most of the decline occurred in the 1990s.

begin

in decline
The industry is still in decline.

on the decline
His career has been on the decline for some years now.

decline in
a steep decline in sales

decline of
the decline of small farming communities

the decline and fall of sth
the decline and fall of a great civilization