It is. And sometimes it isn't. Different people and fields use different definitions. I was referring to the Peano axioms, there zero is considered a natural nu

It is. And sometimes it isn't. Different people and fields use different definitions. I was referring to the Peano axioms, there zero is considered a natural number. On the other hand I referred to the fundamental theorem of arithmetic, there it is more convenient to exclude zero so that one can say that every natural number has a unique factorization into primes. And this is perfectly fine, mathematicians can cope with the fact that there are two different sets that different people call the natural numbers.