The term 'Triangular Greek Letter' is referring to the letter DELTA (Δ), which is the fourth letter of the Greek alphabet. DELTA is a triangular letter, and it is used in mathematics, science, and engineering to represent change in some quantity. It is also used to denote the difference between two values, such as in the mathematical expression Δx = x2 - x1. In addition, DELTA is often used to represent the area of a triangle in geometry.