A title of respect used for people in very high office — ambassadors, some presidents and governors, senior figures in the Catholic church. You almost never see it alone; it comes wrapped in a possessive. Speaking TO the person: Your Excellency ("Thank you, Your Excellency"). Speaking ABOUT them: His Excellency or Her Excellency, usually before the name — His Excellency the Ambassador of Japan. For more than one, the plural is Excellencies, and "Your Excellencies" is how you open a speech to a room of them. Capital E, always. This lives in a narrow world: diplomacy, formal invitations, state ceremonies, and the letters that go with them. If you ever meet it outside that world, it is being used as a joke about someone acting too important. Ordinary heads of government are usually not addressed this way — a British prime minister is not Your Excellency.