The principle that two identical fermions can't have the same position and velocity (to within the limits of the Heisenberg Uncertainty Principle). See also: Heisenberg Uncertainty Principle, fermion, Debriefing
no more than two electrons can occupy an atomic orbital; these electrons must have opposite spin. (see Hund's rule, Aufbau principle) peptide bond
The requirement for electrons (or other fermions) that the wavefunction for a collection of such particles is antisymmetric under interchange of the position and spin coordinates of any pair of identical particles. In the case of noninteracting or uncorrelated particles, it leads to the requirement that at most, one particle can be in each single-particle state.