Well defined matroid invariant

Question

I'm currently studying Gordon/McNUlty book Matroids: a geometric introduction. Chapter 9 is the Tutte polynomial and corank nullity polynomial

Theorem: For all matroids $M$, the Tutte-polynomial is an evaluation of the corank-nullity polynomial: $t(M;x,y)=s(M;x-1,y-1)$. In particular, the Tutte-polynomial well defined matroid invariant.

Question: In this particular case what do they mean well defined?

Is it, every given matroid has a unique tutte-polynomial? (we later learn that the converse is not true). Or that isomorphic matroids will result in the same transformation. $M_1 \cong M_2$ then $f(M_1)=f(M_2)$?

Answer 1

In this case the fact that the Tutte polynomial is a well-defined invariant means that the Tutte polynomials of isomorphic matroids are equal.