The wave function symbols PSIS refer to the Pauli spin matrices, which are a set of three 2x2 matrices that are used in quantum mechanics. These matrices are named after Wolfgang Pauli, an Austrian physicist who developed them in 1927. They are used to describe the spin of a particle and its associated wave function. The Pauli spin matrices are also known as the spin-1/2 matrices, as they represent the spin of a particle with a spin quantum number of 1/2.