For those who have yet to experience the joys of calculus: dx notation is used in calculus (specifically, it is the primary component of Leibniz notation of derivatives). Roughly translated, dx refers to the change in variable X. You most often encounter dx in the format dy/dx : the change in variable y over change in variable x, colloquially as rise over run: the slope of a curve at any given point. dt is used, more specifically, to commonly refer to either change in time or change in temperature (both being represented, often, by the letter t). You see this extremely often in chemistry, though it is often represented there by delta (e.g. delta-t, delta-G, and so forth). (In fact, that's an easy way to remember what delta-x and dx mean: they're effectively the same thing.)