What makes a retro game “retro”?

It’s a question that I’ve been pondering over for some time.  When does a video game stop being considered “modern,” and slip into the vague description of “retro,” or “old-school?”

I’ve heard Halo: Combat Evolved be called a retro game.  My first thought is “That’s crazy!”, as it really does not feel all that long ago since the original Halo was released. 

Retro?  Seriously?

But upon further consideration, maybe there is certain amount of years that a game must be out in order for it to be considered retro.  Let’s say, 10 years?

This would mean that games released in 2000 and earlier are retro.  In 2000, we had Tony Hawk’s Pro Skater 2, Jet Grind Radio, Legend of Zelda: Majora’s Mask, and the Playstation 2 launch, among other notable releases.

Maybe it’s just me, but honestly, I can’t see these games (or consoles) be considered retro.  This is due to the fact that it felt all too recent that I was playing these games, and besides, the mechanics of these games are still prominent today.

So scratch that.  Let’s move on to graphics, and see if it plays a role in a game’s retro status.

I can immediately recognize the Sonic the Hedgehog series on the Genesis as retro, but only on the basis of it being a 2D sidescroller with pixelated sprites.  The question is now: Are 2D games the definition of retro?

This just screams "Retro!"

In that case, would Sonic the Hedgehog 4 be a retro game, even though it will soon be released?  What about the other new releases of sidescrolling games on the Xbox Live Arcade and the Playstation Network?

Maybe there is not a concrete definition of a “retro game.”  Since I’m having trouble defining “retro” in technical terms, perhaps it is a matter of personal preference.  Everyone has their opinions; I don’t consider the original Halo retro, yet others do.

“Retro” is a term that is open for intrepretation.  What is your understanding of a retro game?