Precisely to define applied mathematics is next to impossible. It cannot be done in terms of subject matter: the borderline between theory and application is highly subjective and shifts with time. Nor can it be done in terms of motivation: to study a mathematical problem for its own sake is surely not the exclusive privilege of pure mathematicians. Perhaps the best I can do ... is to describe applied mathematics as the bridge connecting pure mathematics with science and technology.

Applied mathematics is, by definition, interdisciplinary. See especially: Mathematics, Statistics and Probability, Computer Science and Engineering, Psychology, and Economics.