This chapter establishes fundamental definitions essential for professional discourse across disciplines. It emphasises the importance of clear, precise definitions in professional communication, distinguishing this from metaphorical or literary language. The chapter introduces two key concepts: ‘unities’ (entities with physically perceivable boundaries) and ‘constructs’ (human-created concepts for understanding and interacting with the world). A comprehensive typology of constructs is presented, organising them into three interrelated categories: elementary (foundational), primary (addressing who, what, when, where, and how), and supportive (interpretative, addressing why). The chapter examines how these categories interact and complement each other, particularly highlighting the role of physically unperceivable constructs in bridging observable and unobservable aspects of reality. Special attention is given to the challenges and complexities of defining and working with abstract and inferential constructs, both in scientific and social contexts. Through this framework, the chapter provides professionals with a structured approach to understanding and communicating about both tangible and intangible aspects of their work, while acknowledging the dynamic nature of definitions in professional practice.