This is mathematics, not physics. You can define stuff that don't "really" exist in the world, but that work for the purpose of calculating stuff.
"randomness" has as much right to exist in a mathematical sense as a "point" or a "plane".
"randomness" has as much right to exist in a mathematical sense as a "point" or a "plane".
As far as how randomness is defined I believe that might be field dependent but this is me getting out of my depth (I've only taken a few courses on combinatorics).