delta is the forth letter of the greek alphabet and is commonly used in mathematices, computer science and earth sciences. in the system of greek numerals it has a value of 4. the greek alphabet has an important role in the nomenclature of astronomical bodies and subatomic particles. . delta is the forth letter of the greek alphabet and is commonly used in mathematics, computer science and earth sciences. in the system of greek numerals it has a value of 4. the greek alphabet has an important role in the nomenclature of astronomical bodies and subatomic particles.