catastrophic, considerable, dramatic, drastic, massive, marked, precipitate, precipitous, rapid, serious, sharp, significant, steep, substantial | gentle, gradual, modest, slight, slow | continuing, progressive, steady
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

national

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

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

arrest, halt, stop
We must halt this decline in health services.

prevent | accelerate | see
We have seen a sharp decline in educational standards over recent years.

in ~
The motor industry is still in decline.

on the ~
This area has been on the decline for some years now.

~ in
a steep decline in sales

~ of
the decline of British farming

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

politely | absolutely
I absolutely decline to discuss my dealings with him or anyone.

considerably, dramatically, drastically, markedly, sharply, significantly, steeply
The economy has declined sharply in recent years.

a little, slightly | fast, rapidly
The market for these products is declining fast.

steadily | gradually, slowly | further

by
Profits declined by 6% this year.

from, to
The number of full-time staff has declined from 300 to just 50.

decline in importance/numbers/size
This section of the market has slowly declined in importance.