Delta is the symbol used to denote change in mathematic and scientific equations. For example, students are taught that velocity = change in position / change in time. That equation is often given as v=(x-x)/(t-t) [with the first x and t having subscript 2 and the second x and t having subscript 1 to denote that they are two discrete location and time coordinates, but I can’t figure out how to get subscript characters in WordPress!] This, of course, is more simply written as v=∆x/∆t. In other words, a delta represents not some nebulous concept of change but a specific, measurable change between two distinct points in time (or space).