decline

Decline means to refuse something, slope downward, become less or to sink from view. (verb)

An example of decline is someone saying no when they've been asked out to dinner.
An example of decline is a ski run.
An example of decline is the unemployment rate going from 9% to 7.5%.
An example of decline is the sun setting in the West.

A decline is a lessening or failing. (noun)

An example of decline is the collapse of the Roman Empire.
An example of decline is the quality of a glass of milk as it starts to sour over time.
An example of decline is the failing health of someone with cancer.

See decline in Webster's New World College Dictionary