A capital Greek letter Psi commonly used to represent wave functions in quantum mechanics and probability distributions.
Examples
Wave function in quantum mechanics
\Psi(x,t) = A\sin(kx - \omega t)
Eigenfunction notation
\hat{H}\Psi_n = E_n\Psi_n
Statistical mechanics partition function
\Psi(\beta) = \sum_n g_n e^{-\beta E_n}