The term GREEKS' D is referring to the Greek letter DELTA, which is a triangular letter in the Greek alphabet. It is the fourth letter of the alphabet and is represented by the symbol Δ. DELTA is often used to represent change in mathematics and science. In physics, it is used to denote the difference between two values. In chemistry, it is used to represent the energy difference between two states of a system. In mathematics, it is used to denote the difference between two functions.