This paper is devoted to a new topological invariant for symbolic dynamics. One of the classical ways of describing a symbolic dynamical system is by using forbidden words. The authors introduce an equivalence relation \(\simeq\) between functions from \(\mathbb{N}\) to \(\mathbb{N}\). Using forbidden words, the authors prove that any equivalence class of the function that counts the minimal forbidden words of a system is a topological invariant of the system. With the help of the new invariant, the authors show that two systems associated to Sturmian words having ``different slope'' are not conjugate.