noun
BAD : Those who bend the rules are reported to the authority.
GOOD : Those who bend the rules are reported to the authorities.
BAD : The authority had refused them even their basic civil rights.
GOOD : The authorities had refused them even their basic civil rights.
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To refer in a general way to the officials who make all the important decisions in a country, use the authorities : 'The authorities estimate that nearly 100,000 immigrants have entered the country illegally.'
As a countable noun, authority refers to an official group or organization that controls an area or activity: 'You can claim housing benefit from your local authority.'