<p>This study investigated the impact of discussing mathematical events on in-service Mathematics teachers’ professional noticing of geometric definitions. Fifty-two teachers engaged in discussions in which they analyzed students’ thinking regarding rhombus definitions and the teachers’ noticing skills were assessed through pre- and post-tests focusing on rectangle definitions. The analysis, which included qualitative coding and frequency analysis, revealed significant improvements in teachers’ abilities to attend to, interpret, and respond to student definitions following the discussions. Specifically, following the discussions, teachers demonstrated enhanced recognition of essential attributes, more nuanced understanding of student misconceptions, and a shift toward student-centered teaching strategies. The discussions fostered a deeper awareness of the logical structure of definitions and the importance of counterexamples, leading to a more interconnected and refined noticing framework. Notably, teachers applied these refined skills to rectangle definitions, demonstrating transferability. This study underscores the efficacy of integrating mathematical-event discussions into teacher training to develop robust professional noticing skills. Future research should explore the long-term impact and broader applicability of this approach.</p>