The Zeeman effect is the splitting of atomic spectral lines caused by application of an external magnetic field. Pieter Zeeman observed this effect in 1896 and was awarded the Nobel Prize in 1902, along with Hendrik Lorentz, for explaining the phenomenon.

All of the atomic states with the same energy are said to be degenerate. The Zeeman effect is one of a number of mechanism that lift the degeneracy in energy. That is, there are many mechanisms that give the degenerate states slightly different energies. Each level is composed of states that are degenerate with respect to total angular momentum J until an external magnetic field is applied. When an external magnetic field is applied to the system these states are no longer degenerate in energy but are split.

The Normal Zeeman effect was the first attempt to explain the behavior of the spectral lines (energy levels) in a strong magnetic field. It only included the angular momentum of the atomic electrons. However, the number of energy levels predicted by this first attempt were different than those that were measured. This result was known as the Anomalous Zeeman effect. Later, the effect of electron spin was included into the theory, and the measured levels matched the predicted levels.