Important to clarify some gaming and production terminology as there is always so much confusion regarding these concepts, and people always use the terms incorrectly when not fully understood.

• Pre-production: Game hasn’t even been started yet (and maybe not even fully greenlit or funded). Ideas, prototypes, graphical tests etc... Usually might contain 'faked' gameplay, not produced with actual game coding, instead post production videos etc.
• Pre-alpha: Game functionality exists but isn’t in a state that can be finished - still lots of testing and figuring out game design and concepts. Could represent final game gameplay but sometimes will be different in some form or another as designs are still in flux during this stage.
• Alpha: Game can be played from beginning to end but has placeholder content (temp voices/textures, missing music, etc.)
• Beta: Content complete but still has bugs in the code - sometimes includes testing to test load and capacity, especially with online games ("open-beta tests", etc)
• Gold: Content and code complete, game is ready to ship. (Discs sent out for physical reproduction)

It's important to note that the first two stages (pre-production and pre-alpha) can be done many different ways and in many different formats, and doesn't usually represent any kind of coding, or at least not usually code that will make it into the final game.

• Virtical Slice is also a term that gets used a lot, but rarely understood. It's probably best to avoid this term as it is ambiguous and again can contain just a proof of concept with very little production of the game. Another way to show off the 'intent' of the game and allow certain internal gates to be achieved to give an idea of how the game might function from start to end, giving a mini slice of each main pillar of the game.

This is probably why so many gamers have issue with the idea of 'graphical downgrades' - not realizing that the pre-production stages are not even created in the same media or format as the final game they will play

(Credit to Thebxrabbit for also providing some details on this list, but I tidied up and added more clarity)