Delta (letter) has use writing
Change of any changeable quantity, in mathematics and the sciences (more specifically, the difference operator); for example, in  the average change of y per unit x (i.e. the change of y over the change of x). Delta is the initial letter of the Greek word διαφορά diaphorá, "difference". (The small Latin letter d is used in much the same way for the notation of derivatives and differentials, which also describe change by infinitesimal amounts.)
The Laplace operator: