exclusion principle, physical principle enunciated by Wolfgang Pauli in 1925 stating that no two electrons in an atom can occupy the same energy state simultaneously. The energy states, or levels, in an atom are described in the quantum theory by various values of four different quantum numbers; the exclusion principle holds that no two electrons can have the same four quantum numbers in an atom. One of these quantum numbers describes one of the two possible directions for the electron's intrinsic spin. As a result of the exclusion principle, two electrons that are in the same energy level as described by the other three quantum numbers are differentiated from each other because they have opposite spins. This principle applies not only to atoms but to other systems containing particles as well, and it applies not only to electrons but also to a large class of particles collectively known as fermions (see elementary particles).