Linked from
The 64 pages that link to Zeeman effect, each with the reason it gives.
Landé g-factorRelated: The Landé factor sets the linear energy shift of a level in a weak field.
Pieter ZeemanBroader topic: This is the phenomenon Zeeman discovered and measured.
Pauli equationRelated: The equation’s spin–field term produces magnetic energy shifts.
Hale's lawRelated: Hale used its signatures in sunlight to detect magnetic fields in sunspots.