- The collapse of matter under very intense gravitation. The two main types are neutron and electron degeneracy. Electron degeneracy is the condition found in white dwarf stars of less than 1.4 solar masses. Their material collapses so completely that only the repulsion pressures which prevent two electrons from occupying the same niche in the material holds it up at all. Neutron degeneracy applies for larger masses where the electrons are crushed into the nuclei of atoms, turning virtually the whole star into a mass of neutrons. Then the same pressure, this time acting upon the neutrons, allows the material to become stable as a neutron star of immense density.
- In a resonator, the condition where two or more modes have a single resonant frequency.