Named after Willis Lamb: an anomalous difference in energy between two electron orbitals (2S₁/₂ and 2P₁/₂) in the hydrogen atom.
It was precisely measured in 1947 in the Lamb–Retherford experiment on the hydrogen microwave spectrum.
The measurement provided the stimulus for renormalization theory, and Hans Bethe's 1947 calculation of the shift revolutionized quantum electrodynamics.