What is the Mathematics in Mathematics Education?

In this paper I tackle the question What is the mathematics in mathematics education? By providing three different frames for the word mathematics.1. Frame 1: Mathematics as an abstract body of knowledge/ideas, the organization of that into systems and structures, and a set of methods for reaching conclusions.2. Frame 2: Mathematics as contextual, ever present, as a lens or language to make sense of the world.3. Frame 3: Mathematics as a verb (not a noun), a human activity, part of one's identity.After introducing the frames and examining their distinction and their overlap, I discuss their implication with respect to student-centered classroom, context, and culture.