<p>The dynamic relationship between language and perception is well documented: we seek language to describe what we perceive and, in addition, what we perceive is driven by the language we have at our disposal. However, little literature in the field of teacher noticing has directly investigated the relationship between what teachers notice and the language they have available to describe what is noticed. This paper examines this relationship by exploring the mutual development of mathematics teachers’ noticing abilities and the language they use to describe teaching and learning. This study draws on video recordings and written reflections from a class of pre-service secondary mathematics teachers in the U.S. who, over ten weeks, examined video records from classrooms, discussed classroom noticing, and sought to deepen their understanding of students’ mathematical thinking. In particular, the pre-service teachers and their instructors collaboratively constructed a definition of the term <i>substantive student thinking</i> and used this developing construct as a tool in their noticing. Through our qualitative analysis of records from this course, we document different ways in which language influenced noticing, noticing influenced language, and language and noticing co-developed as PSTs developed and applied their understanding of the previously undefined term, substantive student thinking.</p>