Software engineering faces the challenge of developing and maintaining systems that are highly variable in space (concurrent variations of the system at a single point in time) and time (sequential variations of the system due to its evolution). Recent research aims to address this need by managing variability in space and time simultaneously. However, such research often relies on nonuniform terminologies and a varying understanding of concepts, as it originates from different communities: software product-line engineering and software configuration management. These issues complicate the communication and comprehension of the concepts involved, impeding the development of techniques to unify variability in space and time. To tackle this problem, we performed an iterative, expert-driven analysis of existing tools to derive the first conceptual model that integrates and unifies terminologies and concepts of both dimensions of variability. In this paper, we present the unification process of concepts for variability in space and time, and the resulting conceptual model itself. We show that the conceptual model achieves high coverage and that its concepts are of appropriate granularity with respect to the tools for managing variability in space, time, or both that we considered. The conceptual model provides a well-defined, uniform terminology that empowers researchers and developers to compare their work, clarifies communication, and prevents redundant developments.