The Orange Order (or Loyal Orange Institution, to give it its official title) is a Masonic-style society which was founded in Ireland in 1795, during a period of sectarian conflict in the country, to defend the Protestant religion and uphold the Protestant rule of the British monarchy. Its members, known as Orangemen, are nowadays mostly to be found in Northern Ireland, though there are also small numbers in the Republic of Ireland, in Scotland, in some Commonwealth countries, and in the United States. Clearly, all members or supporters of the Orange Order are loyalists (since they are loyal to the British monarchy) and unionists (since they believe in the union of Northern Ireland with Great Britain), though they need not be members of one of the Unionist political parties (i.e., the Democratic Unionist Party (DUP) or the Ulster Unionist Party (UUP)). (Equally clearly, the converse is not true: some loyalists and/or unionists are not members or supporters of the Orange Order.)
Orangemen are most visible on 12th July each year when, wearing orange sashes, carrying banners, and often accompanied by fife-and-drum bands, they parade through the streets to commemorate the Battle of the Boyne, at which in 1690 the Protestant King William III of England (1650-1702) defeated the deposed Catholic King James II.
The Orange Order is named after King William, who was born Prince of Orange, became the ruler (Stadtholder) of various principalities in the Netherlands in 1672, and in 1689, after the deposition of King James II, was invited with his wife, Mary, the daughter of James II, to come to England to assume the British throne. The deposed King James fled to Ireland, where his forces suffered a series of defeats in the so-called Williamite War in Ireland (1689-1691). Queen Mary died in 1694, but William continued to reign as sole monarch until his death in 1702.
Orange, the territory of which William was born Prince, was a small state in Provence in the south of France. In 1163 it became part of the Holy Roman Empire and in 1544 was acquired by the Dutch monarchy, the Royal House of Orange-Nassau, who continued to hold it until 1713, when it was ceded to France under the Treaty of Utrecht.