Chiefly with capital initial. A soldier in the Union Army during the American Civil War (1861–5); (in plural) Union forces. Now historical.
In sense 13a(b), with the sense ‘of or belonging to the side in the American Civil War (1861–5) which opposed the seceding Confederate states’, as…
In plural. In the American Civil War (1861–5): an infantry brigade of the Union Army noted for their black-coloured hats. Now historical.
(a) U.S. Union (as opposed to Confederate) troops in the American Civil War (1861–5); (b) sailors in the navy; (c) policemen (now the usual sense).