The authorities are the people who have the power to make decisions and to make sure that laws are obeyed.

An authority is an official organization or government department that has the power to make decisions.

Authority is the right to command and control other people.

If someone has authority, they have a quality which makes other people take notice of what they say.

Authority is official permission to do something.

Someone who is an authority on a particular subject knows a lot about it.

If you say you have it on good authority that something is true, you mean that you believe it is true because you trust the person who told you about it.