So, i must have missed something but what does it mean to be "Pure human" in the Fallout Universe?

I ask because from what i understand it means being born in a Vault or another place secure from the outside world and it's background radiation and mutations.

Now comes the question,to be a pure human you must live your whole life in a controlled environment without exposure to the outside? This would mean some of the Enclave itself aren't pure humans.

If not,and i think that's not the answer,what is it?

Not sure where the term "Pure Humans" comes from, the Master refereed to them as "Prime Normals" and the Enclave have never used the term. As with real-life radiological incidents, exposure to radiation creates mutations in DNA, genetic illnesses and anomalies, higher percentages of cancers and so on; these are passed, as genes, through the survivors of the war. Those whom were kept safe from the nuclear war and radiation since then are genetically on-par with pre-war humans and free from radiation induced genetic factors unlike those passed through great war survivors descendants.