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President, presidency, presidential

Capitalize as a title when it immediately precedes a personal name; otherwise lowercase, including appositives.
Avoid abbreviating.

            President Barack Obama but U.S. president Barack Obama
            President-elect Barack Obama; the president-elect
            president of the United States, the president, a president, his presidency
            presidential powers
            president of the Confederacy
            President and CEO Gary E. Knell
            president of the National Geographic Society, the president, his presidency
            president of France, the president
            Jane E. Vessels, president of CBS; President Vessels; the president of CBS

See also: Vice President, Vice Presidency, vice presidential.