From AMS Glossary
A shift in the energy levels of an isolated atom or molecule as a consequence of an external magnetic field.
The energy levels of an atom (or molecule) depend on the internal forces that electrons and nuclei exert on each other as well as on any external forces (e.g., an electromagnetic field). An observable consequence of the Zeeman effect is the shifting and broadening of spectral lines. The Zeeman effect is the magnetic analogue of the Stark effect.