The latin letter is used in algebra as a variable, often to represent distance. The symbol also appears in calculus expressions to represent the differential in expressions like this:
In plain language, this expression represents the integral of the function with respect to . The “with respect to” statement corresondings to the latin letter and the variable following the symbol is the variable being integrated over. See integral notation for typesetting and more.
The capital Latin letter D.
The partial represents a partial differential.
The distance between two points, in one dimension, is given by the absolute value of the difference between the two values.
The distance between two points, in two dimensions, is given by solving pythagorean's theorem for the length the hypotenuse of the right triangle formed by the two points.