Commonly stated as the
common sense notion that two things can not be in exactly the same place at exactly the same time. In physics it is given the specific meaning that no two
identical fermions can have the same
quantum numbers at any given time. This accounts for the fact that
electrons in
atoms stack up in progressively higher
energy levels, which in turn accounts for the fact that
chemistry is
interesting.
If two identical particles are forced to try to have the same quantum numbers they will respond with a force called the degeneracy pressure, or the Pauli Repulsion. There is a type of star, called a degenerate white dwarf, that is held up entirely by this force.