If its on one platform (say PS4) but missing on another say (XB1 or Switch), then its exclusive.

It used to be tied to a brands generational hardware.

Now its tied to a brands compatible hardware and/or online service.

The future will be tied to a brands optional hardware and necessary online service(s).

Personally if a game comes out on PC _and_ on a console, I usually buy it on the console. I consider console exclusives, as a collector, the better buy. The exceptions are games that have a large modding community, in which case I buy for both the PC and the console.

If I remember correctly though, mobile (phone/tablet) game sales > PC game sales > console (console/handheld) game sales.

Exclusive, to me, is just that; exclusive. As in one platform only, if it's available on more than one platform it's not exclusive. Nintendo exclusive, Xbox exclusive, PS exclusive, PC-Xbox exclusive, they all entail availability on more than one platform, which isn't an exclusive. "Console only" or "Nintendo only" would be a better term, from a purely semantics perspective anyway.

exclusive has always meant for me that I can´t play the game on a competitors console. PC is a universal platform that I have never counted since it doesn´t have a publisher/creator behind it that competes with the traditional consoles and has so many variations Tech-wise that it can never be defined as belonging to a specific generation.

This was also true during the 90´s and even went so far that I defined exclusive in terms of features. For example, I always held Street fighter 2 on Megadrive above its SNES counterpart simply because it was faster =). So, exclusivity has IMO always been a way to just differentiate your favored console against the direct competition.

Depends on the context someone is talking about.

This (almost).

We have 3 simple expressions covering all console games (these can all be timed of course).

1. Exclusive game - game available on only 1 console (Uncharted 4, Halo 5, Splatoon)
2. Console exclusive game - game also available on PC/arcade/etc. but not on any competing consoles (Street Fighter V, Dead Rising 3, Pokkén Tournament)
3. Platform exclusive game - game available on only 1 platform (e.g. Sony) of consoles (e.g. PS4, PS3 and Vita) and not on PC or any competing consoles (Persona 5, Forza Horizon 2, Zelda BOTW)

One could use the term "Microsoft exclusives" about games such as Dead Rising 3 (also on Steam) or Titanfall (also on Origin) since PC is technically also a "Microsoft platform" but personally I'd only consider using that if the PC release was limited to Windows 10 Store (e.g. Forza Horizon 3) and not on Steam/Origin/etc. And even then I'd probably just refer to them as console exclusive games.

You can make dedicated machines though, hence the Nvidia and AMD lines. Office PC's don't even need the kind of *gaming* GPU's that those two vendors sell.

Also Steam doesn't rely on deals, there are deals on other clients like Origin, Uplay, Battle.net, GoG, but even then otuside of those, people still buy games at launch just like any other person does on other systems.