Research in the out-of-school time (OST) field confirms a strong connection between professional development for staff and positive experiences and outcomes for youth. In recent years, the OST community has invested significant time and money into researching, creating, implementing, and evaluating professional development activities for OST staff. The goal of this paper is to begin a conversation in which a national collaborative of OST organizations can agree on a common set of professional development definitions. This article presents a set of commonly used terms in the OST field, provides examples of how the OST community has adapted and refined these terms, and proposes a series of OST professional development definitions.