Excellency is a honorific style that is given to members of a certain organization or state, usually a president or governor. Once a person gets the title of "Excellency" it is usually held by that person for the rest of their life though this is not always the case. For example some countries presidents get the title of "Excellency" but only hold it for their time in that office.
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Their Excellencies the Lords Justices of England, for the administration of the Government during the absence of the King by Robert White
Excellency Facts for Kids. Kiddle Encyclopedia.