to announce something or to state something firmly, clearly or officially
While the UN doesn't declare war, there have been several recent cases of UN actions that can be regarded as 'lawful authorisation'.
to tell an official person the value of goods you have bought, or the amount of money you have earned (eg because you might have to pay tax, be eligible for benefits etc)
Mr Burke had failed to declare an income of £91,980. His lawyer said Mr Burke was now tax compliant and urged the judge not to jail him.