Adaptive learning and personalization have long been of great interest to learning designers and educators, and recent technological advances that have opened up a range of new possibilities for adaptivity. However, we lack clear definitions of the terms adaptivity and personalization, and the theoretical and empirical soundness of implementations of corresponding systems varies greatly. We therefore first provide definitions for key concepts related to adaptivity. We then discuss what variable systems should adapt for, how these variables can be measured, and what modifications to the learner experience can be made based on these variables. We propose a taxonomy of adaptivity that distinguishes adaptivity based on cognitive, emotional, motivational, and social/cultural variables, and that defines types of adaptivity at a macro-level and a micro-level.