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In physics and chemistry, a metastable state is a specific excited state of an atom, nucleus, or other system that lasts longer than typical excited states but generally has a shorter lifetime than the ground state, which is the lowest, most stable energy level.

The more population of atoms in the metastable state (above ground state) contributes more in stimulated emission than stimulated absorption in laser action. More number of atoms in metastable state is achieved. This causes a phenomenon known as population inversion, which is required to generate lasers due to its comparatively long lifetime. The number of electrons congregating in the metastable state is greater than those in the ground state in such a situation. Because a metastable condition has a long lifetime, this results in a cascade of stimulated emissions.

Since non-metastable states have a substantially shorter duration,$(~{10}^{-8})$ population inversion is far less likely to occur.