x y is by default interpreted as CirclePlus[x, y].
Used in mathematics for various operations related to addition, such as direct sum and addition modulo two.
Also sometimes used to indicate a vector pointing into the page.
See The Mathematica Book: Section 1.10.8 and Section 3.10.4.
See also: \[CircleTimes] , \[CircleMinus] , \[Xor] .