Infix operator with built-in evaluation rules.
x y is by default interpreted as Union[x, y].
The character is sometimes called "cup"; but see also \[Cup] .
See The Mathematica Book: Section 1.10.4, Section 3.10.1 and Section 3.10.4.
See also: \[Intersection] , \[SquareUnion] , \[UnionPlus] , \[Cup] , \[Vee] .