The distribution of a game onto multiple platforms.
The decision to release a game onto multiple platforms can have a significant impact on the design of the game. The ability to fully capitalize on exclusive abilities of one platform is exchanged for a much larger potential audience to be able to experience roughly the same gameplay.
Questions that deal with how to effectively develop a game to run cross-platform fall under the purview of this tag. This also can apply to concerns about what technology will function when used in cross-platform pursuits.