A special technique for obtaining an approximation to the solutions of the one-dimensional time-independent Schrödinger equation, valid when the wavelength of the solution varies slowly with position; also known as the WKB method. It is named after G. Wentzel, H. A. Kramers, and L. Brillouin, who independently in 1962 contributed to its understanding in the quantum-mechanical application. It was however, studied earlier by J. Liouville (1837), Lord Rayleigh (1912), and H. Jeffreys (1923). It is also called the BWK method, and the JWKB method, the classical approximation, the quasiclassical approximation, and the phase integral method.