
Platform
This refers to the hardware on which the game will be played or a particular distribution platform. Hardware such as PCs, games consoles, mobile devices, and virtual reality headsets are considered platforms, but also specific environments such as Facebook or iMessage are platforms themselves. Each platform brings a unique set of requirements and opens up very different possibilities, both on the technical and creative sides.
Choosing the right platform for your game has to be one of the earliest decisions. The platform on its own can have an immense influence on the game design, from the target audience and business model to controls, user interface, and performance requirements. Platform choice in itself sets certain expectations towards the game you're making. There's a good reason why match-three puzzle games are not swarming the console market.
From an audience perspective, a card solitaire game may work really well on smartphones but is unlikely to be as popular on consoles. Similarly, but from a hardware angle, a strategy game that relies on precise controls provided by mouse and keyboard might not translate very well to the console or handheld input methods.
There are many unspoken rules to game design, and knowing when to break them is part of the art. Introducing a complex strategy game to the smartphone market and tackling the challenges of the platform head-on might result in a very innovative and interesting product. But remember, such experiments are risky, so make sure you know the rules before you try to break them!
Each platform has also a very tangible impact on how the game will be designed. The most evident example is about input controls: an action game would pose completely different challenges on a mobile device than on a console controller...
Those difference in design are not limited to the platform technology; the way the player expects to interact with games on different platforms is important too. A mobile game would be usually characterized by short sessions where the player is able to complete the whole game loop in just a few minutes, while a PC game could require the same amount of time just to go through a single cutscene.