Wow. There's actually a definition within law, which I cannot cite for you because it goes on and on. For our purposes, any veteran who served on active duty prior to 1980, as long as they served and received an other than dishonourable discharge, they're going to be eligible for burial or for burial benefits. After 1980, a 24-month rule was added in, so the veteran had to serve 24 months to be eligible for burial.
Is this what you're looking for, or are you looking for a general, overall definition? A veteran is somebody who served in the military, basically.