(1650-1702) the king of Britain and Ireland from 1689 until his death. He was also called William III. He was married to the daughter of King James II, and was asked by James's enemies to become king instead of him. William and his Protestant army beat James and his Catholic army at the Battle of the Boyne, and for this reason Protestants still admire him in Northern Ireland and sometimes call him 'King Billy'. William's wife, Queen Mary II, had equal power, and people usually talk about the reign of William and Mary.
Definition from the Longman Dictionary of Contemporary English Advanced Learner's Dictionary.
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