While research in teacher professional development (PD) has focused on numerous topics, not much is known about how teachers understand the notion of PD. Studying this topic is important because, for PD to be effective, all parties involved in its design and implementation need to have a common understanding of the scope and purposes of PD. We asked 315 Singapore primary school music teachers to define PD using their own words. Descriptive statistics, cluster analysis, and cross-tabulations were used to analyze the definitions and the extent to which these differed according to certain demographic variables. We found that teachers' definitions were not entirely consistent with those proposed in the educational literature, as teachers referred to a narrower scope of ideas regarding the content, facilitation type, format, and outcomes of PD. For example, while the ultimate goal of PD according to scholarly definitions is enhancing student learning, teachers seem to primarily understand PD as a means to improving themselves. No differences were identified when teachers' definitions were compared according to variables such as age, teaching experience, designation, and specialization. Based on our findings, we conclude that it is essential to help teachers develop broader understandings of what PD means and entails.