As far as I can tell the rest of the world i.e to those people who have no interest in video games at all, retro means anything from the early to late '70s. Brown Ford Capris, flowery wall paper and bell bottom flares.
I don't know what most of you were playing in the 70's but I know for a fact it wasn't the ZX Spectrum or the CBM64....
Gamers have taken the word "retro" and used it to decribe an older machine they have memorys of playing when younger. As the world moves forward some peoples first experiance of video gaming is the Megadrive or the Amiga, in some case's (on this very forum) the first machine people had were PS1's and Saturns ( poor sods

To me Retro means any machine older than a PS2/XBOX at the moment, but as time moves foward and the PS4 and XBOX 720 are released then this invisible line moves along and then starts to included the PS2 and XBOX... But that wont be for a long time yet...