In the presence of magnetic fields, spectral lines are split. The pattern and amount of splitting are a signature that a magnetic field is present, and of its strength. The splitting is associated with what is called the orbital angular momentum quantum number L of the atomic level. This quantum number can take non-negative integer values. The number of split levels in the magnetic field is 2 * L + 1.

Atomic physicists use the abbreviation "s" for a level with L=0, "p" for L=1, "d" for L=2, and "f" for L=3. It is also common to precede this designation with the integer principle quantum number n. Thus, "2p" denotes a level that has n=2 and L=1.

In the adjacent example the lowest level is an "s" level, so it has L=0 and 2L + 1 = 0, and it isn't split in the magnetic field, while the first excited state has L=1 ("p" level), so it is split into 2L + 1 = 3 levels by the magnetic field.

The Zeeman effect can be interpreted physically in terms of the precession of the orbital angular momentum vector in the magnetic field, similar to the precession of the axis of a spinning top in a gravitational field.