I have never heard of any such thing, and neither has Google. I suspect you are very confused. Can you give an example of an "undefinable formal system" and say

I have never heard of any such thing, and neither has Google. I suspect you are very confused. Can you give an example of an "undefinable formal system" and say (1) how it satisfies the definition of "formal system" and (2) the sense in which it is "undefinable"?