Assume we're talking about centerfire rifle?
I'd say anything less than 200 yards is short range, between 200 and 500 is mid range, and anything 500+ is long range.
Most important factor I think is a good optic, one that you can turn the dials on the turret and always have them go back to the same zero. I hear alot of people saying you need to have a big heavy bullet going at high velocity but I've shot out to 600 yards with plain old 55gr .223
My current long range gun is a 6.5mm Grendel AR-15, have a 16x fixed power scope on it. Works pretty well, but I could use more practice with it (I'm still a bit of a noob at long range stuff)