Psi is the twenty-third letter of the Greek alphabet: indicating the combination of /ps/ in both the classical and Modern Greek languages. Psi is usually transliterated as “ps”, but due to phonotactic, or pronunciation, constraints of the English language, its sound is often simplified to “s”. As a symbol, Psi has several meanings. Psi has been translated to represent and embody the soul, psyche, and the unknown. Also, the letter Psi is often used in physics as a symbol for a wave function in quantum mechanics. Other usage includes Psi as a symbol for the polygamma function, the reciprocal Fibonacci constant, rare nucleotide pseudouridilic acid, and the planet Neptune.