Almost all gamers have their own idea of what 'retro games' are. The problem is that they frequently don't match when compared to each other, as most people use the term subjectively to describe the games from their own youth. For me, retro used to mean everything from the 8 to 32 bit console eras, and strictly applied to 2D games. For others, the Nintendo 64 or the first Playstation was their first console, and is therefore retro to them. That's always jarring for me to hear, as the shift to 3D graphics in the fifth console generation has always signified to me the end of the retro years.
So what do we do about these discrepancies? First off, we should acknowledge that just using the term 'retro' on its own isn't enough; games have been around long enough for us to need more accurate terminology. There have been seven generally recognized generations of home video game consoles, and distinctions can even be drawn between early and late periods within these generations. However, even without those distinctions, seven generations is too many to keep straight or be useful.
I propose as a solution that we group similar generations together to create four distinct eras of video game history. After each grouping I've given a tentative name to the era, listed the major representative consoles, and given my reasons why they belong together.
Pre-crash: generations one and two
Odyssey, Pong, Colecovision, Atari 2600, etc. ect. etc.
For me, this era of games has always been off of my radar. It was before my time, and the types of games you'd have seen during this era were largely unrefined and still experimenting with what could be done with the existing hardware. At the time, people put up with them due to their novelty, but the thrill wore off when the market became flooded with competitors trying to cash in on the game craze, causing the crash of 1983.
With a few exceptions, these early styles of games haven't survived to the modern day.
Golden Age: generations three and four.
NES, Master System, TG16, Genesis, SNES
The advent of the NES to me signals the beginning of modern gaming. Its controller was the first to use the layout featuring a d-pad on the left, start and select in the middle, and buttons on the right, on which nearly every standard controller since then has been based. Many 2D game genres were either created and/or perfected at this time and a tremendous amount of classic franchises were born, many of which are still continuing to this day.
Silver Age: fifth generation.
Saturn, Playstation, N64
2D games that came out in this era had reached stunning levels of technical excellence, but were becoming rarer and rarer as Nintendo and Sony actively pushed developers towards making games that showed off their consoles' new 3D technology. There were an awful lot of growing pains as game developers had to create all new ways to either bring established genres into the third dimension or create entirely new types of games.
While many 3D genres were created at this time, many would not be perfected until the following generations. Since controllers wouldn't feature two analog sticks until later in the Playstation's life, controlling characters in 3D space was quite awkward in many games, and almost every game had a unique control scheme.
Current: sixth and seventh generation.
Dreamcast, Xbox, Gamecube, PS2, 360, Wii, PS3
Though the term 'current' tragically no longer applies to the Dreamcast, all the other sixth gen consoles are survived by some degree of backwards compatibility, making them still somewhat relevant. The 360 is still running the Xbox Live service started on the original Xbox, and the PS2 still gets the occasional release.
3D gaming found its stride at this time, helped greatly by the designs of current controllers. Barring the Wii and Dreamcast all of them have two analog sticks, making simultaneous control of character and camera easy.
Another major feature of the seventh generation is online marketplaces. The implications and effects are a little too far reaching to address here, but their impact has been significant in allowing smaller developers a shot at major success.
So these are the distinctions that I usually think of when placing a game in history. It's important to note that this was not in any way meant to address arcade, hand-held, or PC gaming, as their histories have followed quite different paths.
I've borrowed the golden and silver age terms from comic fandom, but am not terribly attached to them. I'd love to hear any alternate name suggestions if you have them, or even suggestions for alternate groupings. This list can't be completely objective as we've all had different experiences with different consoles, but I hope it's a step in the right direction.