1. An electronic excitation, observed at low cryogenic temperatures in silicon and germanium, in which an electron-hole Fermi liquid is formed in an unstable condition.
2. A process by which a molecule is excited from a low-lying electronic state to a higher energy electronic state as observed in germanium and silicon at sufficiently low cryogenic temperatures.

It is associated with a liquid-gas phase transition of the charge carriers, and consists of regions of conducting electron-hole Fermi liquid coexisting with regions of insulating exciton gas.