Randomness is a well defined concept. The problem is that one literally can never tell if a deterministic pattern won't someday be identified in some observed phenomena that currently seem random, like radioactive decay.
So, we can define random mathematically, but the most we can say about observations is that they seem random.
From a practical point of view radioactive decay is totally random, but that’s only because we lack the tools to get inside to see exactly what's going on. That may not ever change for us humans, but I wouldn't bet against it. Scientific discovery is all about de-randomizing mysterious things and is based on the assumption that nothing is really random, just currently not understood.