"Generation One" was originally a fan term used to describe all Transformers that came before "Generation 2" and that's basically what I suppose most fans would classify as the "original G1" (so not including reissues, retro-G1 etc.).

The original series of pre-G2 Transformers ran from 1984-1993, so yes, a total of 9 years. The 1993 G1 Transformers were later repackaged as G2 Transformers and so those toys are sometimes referred to by fans as "G1.5" but that term's never been used in an official capacity. Officially those toys are either G1 or G2 depending on which packaging they came in (which is impossible to tell if you get them loose w/o card or box, hence why fans created the G1.5 category).