The Pauli exclusion principle is a quantum mechanical principle formulated by the Austrian physicist Wolfgang Pauli in 1925. In its simplest form for electrons in a single atom, it states that no two electrons can have the same four quantum numbers; that is, if n, l, and ml are the same, ms must be different such that the electrons have opposite spins. More generally, no two identical fermions (particles with half-integer spin) may occupy the same quantum state simultaneously. A more rigorous statement of this principle is that for two identical fermions, the total wavefunction is anti-symmetric.
In contrast, integer spin particles, bosons, are not subject to the Pauli exclusion principle. For bosons, any number of identical particles can occupy the same quantum state, as with, for instance, lasers and Bose-Einstein condensation. (wikipedia)
See Also
Enharmonic Fermion Figure 14.07 - Love Principle: Two sympathetic waves expanding from two points have one coincident centering locus Harmonic Polarity Principle of Polarity Principle of Regeneration