When it comes to technology to me it's something that has seen a niche resurgence after a period of being considered worthless. Here are the stages a generation of hardware goes through
Latest and Greatest - The most recent 2 perhaps 3 generations of hardware which you can generally still get new very easily.
Mainstream - Not the latest, but still useable for most modern tasks. No one will fault you for having hardware like this but might say you need an upgrade
Junk/Recycle - This is the period where hardware is generally worth less then it would cost to even ship it. Most of the time this hardware is sent to the recycler because no one is willing to spend any money on it and it struggles with modern operating systems.
Retro - After significant amount of recycling hardware is starting to get hard to find and some people are starting to feel nostalgic about the fact that hardware is disappearing. Value starts to increase for a niche of collectors or nostalgic users.
Vintage - Hardware is extremely hard to find and it's value might even be more then it was when it was new.
For instance right now I personally feel like it's Pentium 3 era and earlier for PCs that one can truly consider retro. We are at a point where Slot1/Socket370 systems it's a shame when you see one in the trash because they are starting to get harder and harder to find.
Pentium 4, Core2 and anything in between at a point where you can't hardly even give it away (except for a few particular pieces that were really expensive when they were released) and there is so much of it out there that it's just taking up space and lots of them are just being trashed. Now is the time to hold on to some of these they start to become hard to find. Right now they are basically at the Junk\Trash\Recycle stage.
Core i systems still have value but are valued a much less then they were when they were introduced but are pretty much still useable for any modern tasks including most modern games, assuming you have appropriate GPU. This would be mainstream generation.
More recent Core i systems are still holding their value pretty much even used ones and are still on the cutting edge or maybe right behind the cutting edge. The latest and greatest generation.