This paper proposes to define the field currently known as AIED not in terms of the technology used, but in terms of system behavior. Specifically, it is proposed that AIED is the science and engineering of systems that adapt to learners, so as to help bring about effective, efficient, and enjoyable learning experiences. But what, in general, is adaptivity? Intuitively, being adaptive means that the system adjusts the course of instruction in nuanced and effective ways based on learner differences, for example the goals and needs of individual learners and group of learners. It is difficult to state necessary and sufficient conditions for the concept of adaptivity. Instead, I stipulate that a system is more adaptive to the degree that: (a) its design is grounded in a thorough (empirical) understanding of learners in the given task domain, (b) it is appropriately interactive, and (c) it takes into account, in its pedagogical decision making, how individual learners measure up along diff...