An Etymological Dictionary of Astronomy and Astrophysics

1. The state of being multiple, made of several components.
   

2. In atomic and nuclear physics, the number of → levels into which the energy of an atom, molecule, or nucleus splits as a result of
   → Russell-Saunders coupling between → orbital angular momentum and → spin angular momentum. It is given by 2S+1, where S is the total electron → spin quantum number. The multiplicity of an energy level is indicated by a left superscript to the value of L, where L is the resultant electron orbital angular momentum of the individual electron orbital angular momenta.
   

3. In → statistical mechanics, the number of → microstates corresponding to a given → macrostate.

See also: → multiple; → -ity.