PW91, short for Perdew-Wang 1991, is a type of exchange-correlation functional used in Density Functional Theory (DFT). Developed by John Perdew and Yue Wang, it is among the Generalized Gradient Approximation (GGA) functionals, which are designed to improve upon the Local Density Approximation (LDA) by including the gradient of the electron density to better capture the effects of electron correlation and exchange.