A variant form of the Greek capital letter Psi commonly used to represent wave functions in quantum mechanics.
Examples
Wave function in quantum mechanics
\varPsi(x,t) = A\cos(kx - \omega t)
Grand canonical partition function in statistical mechanics
\varPsi(T,V,\mu) = \sum_{N=0}^{\infty} Z_N(T,V)e^{\mu N/kT}
Stream function in fluid dynamics
\vec{v} = \nabla \times (\varPsi \hat{z})