theta is the eighth letter of the greek alphabet, and in the system of greek numerals it has a value of 9. the lower-case letter is used as a symbol in mathematics, geometry, meteorology and genetics. the greek alphabet has an important role in the nomenclature of astronomical bodies and subatomic particles. . theta is the eighth letter of the greek alphabet, and in the system of greek numerals it has a value of 9. the lower-case letter is used as a symbol in mathematics, geometry, meteorology and genetics. the greek alphabet has an important role in the nomenclature of astronomical bodies and subatomic particles.