It is the broken-off tip of a civil war "socket" bayonet's blade. It appears to be from a British-made Enfield bayonet, which was used mainly by the Confederates, but the yankees also imported huge numbers of that kind.
I can't quite be sure from your photos of the still rust-covered bayonet tip whether all three of its sides are "fluted" (concave), or whether one of the three is flat. If all three are concave it is from an Enfield. If one is flat, it is from a (strictly-yankee) Springfield bayonet.