Edit - replaced the word 'pumped' with 'lumped'.
The most recent clear definition I have heard is anyone born between 1980 and 2000. So anyone ~16 and under is a new generation, currently called Gen. Z, but they may get another name soon.
It does seem like the media is absorbing Gen X into millennials now for some reason. I don't know why. Baby Boomers vs everyone else?
My wife definitely has more of the typical millenial characteristics then I, and talking of our high-school experiences it almost seems like we were decades apart rather than just 5 years, though perhaps some of that is that we grew up on opposite coasts.
Wait, I thought those were "digital natives", i.e. never having known life without ubiquitous computers/tablets/cellphones? I've mostly heard the progression as Gen X (born in the 70s), then Gen Y (born in the 80s). After Gen Y it's supposed to be Millennials (born in the 90s/coming of age in the 00s), then Digital Natives (aforementioned "raised by the Internet").
But really, I think they just make it all up to suit their current viewpoint. I'm 38 and I've been told I straddle the line between Gen X and Gen Y. I jammed to Pearl Jam and Alice in Chains in 9th grade while wearing flannel shirts and combat boots, so I'm Gen X...wait I also had an 80s electro revival phase wearing all black the following year, so I'm Gen Y...
You get the picture.
Millennials are the group born in the 80s/early 90s and were introduced the internet at some point while they were growing up, rather than it having always existed as part of their life.
Nobody is really sure what generation Z is like yet, but they are definitely different than millennials.