This chapter introduces a comprehensive glossary developed as part of the COST Action CircularB, aimed at standardising terminology related to the circular economy and the built environment. Acknowledging the conceptual complexity and terminological inconsistencies across various disciplines and practices, this glossary serves as an essential reference to enhance communication, research, and implementation. It encompasses over 400 terms and definitions sourced from European regulations, ISO standards, and expert practices, providing clarity and consistency for stakeholders, including policymakers, practitioners, researchers, and industry professionals. The glossary was designed with the overarching goal of promoting a shared language that fosters collaboration and accelerates the transition toward a sustainable and circular built environment.