Excellency is an honorific style given to certain high-level officers of a sovereign state, officials of an international organization, or members of an aristocracy. Once entitled to the title "Excellency", the holder usually retains the right to that courtesy throughout their lifetime, although in some cases the title is attached to a particular office, and is held only for the duration of that office.
Generally people addressed as Excellency are heads of state, heads of government, governors, ambassadors, Roman Catholic bishops and high-ranking ecclesiastics and others holding equivalent rank (e.g., heads of international organizations). Members of royal families generally have distinct addresses (Majesty, Highness, etc.)
It is sometimes misinterpreted as a title of office in itself, but in fact is an honorific that precedes various titles (such as Mr. President, and so on), both in speech and in writing. In reference to such an official, it takes the form His or Her Excellency; in direct address, Your Excellency, or, less formally, simply Excellency The abbreviation HE is often used instead of His/Her Excellency; alternatively it may stand for His Eminence.