psi is the 23rd letter of the greek alphabet and has a numeric value of 700. the letter psi is commonly used in physics for representing a wavefunction in quantum mechanics. it is also used in biochemistry, pharmacology and computational theory. the greek alphabet has an important role in the nomenclature of astronomical bodies and subatomic particles. . psi is the 23rd letter of the greek alphabet and has a numeric value of 700. the letter psi is commonly used in physics for representing a wavefunction in quantum mechanics. it is also used in biochemistry, pharmacology and computational theory. the greek alphabet has an important role in the nomenclature of astronomical bodies and subatomic particles.