Three political meanings:
(1) elliptical for trades unionism; therefore, support for the political aims of trade unions;
(2) support for the Union (i.e. the North) before and during the American Civil War;
(3) in the UK since 1886, support for the maintenance of the United Kingdom of Great Britain and Ireland (since 1921, Northern Ireland). Although the word in sense (iii) is not attested before 1886, the concept goes back to the seventeenth century. The 1707 Union between England and Scotland was driven, on the English side, by the fear that the Scots might pass an act of succession that would give them a different (and possibly warring) king to the king of England. On the Scottish side, the unionists perceived that an independent Scotland was economically doomed to domination by the much larger English economy. The Irish Union of 1800-1 was likewise driven by strategic arguments on the British side. It was less stable than the Scottish Union, because the large majority of the Irish were Catholic, disenfranchised (a promise to enfranchise them after Union being broken), and not consulted.
Irish demands for Home Rule (what would now be called devolution) led 1886 to the regrouping of British politics into a unionist and an anti-unionist coalition. The centre of gravity of Unionism was always in the Conservative (for much of the time since 1886 it has been officially entitled the Conservative and Unionist) Party. But for most of the twentieth century the Labour Party was also unionist, for a principled and an unprincipled reason. The principled reason was that securing equal living standards throughout a nation implies unionism because it must entail redistribution from rich regions to poor ones, and therefore a central government strong enough to enforce that. The unprincipled reason was that Labour needed the parliamentary votes of its Scottish and Welsh MPs, where they were doubly overrepresented (Scotland and Wales had too many seats, and the plurality electoral system gave them a majority of seats on a plurality of the vote).
Unionism has weakened substantially since 1997. Conservatives care less about the Union as such, now that they hold almost no seats outside England. Labour controls a majority of seats in England and therefore needs its Scottish and Welsh overrepresentation less.