Hi there, I am reading a book regarding fundamental atomic physics, in which it introduces one kind of electronic scattering called Kapitsa–Dirac effect. I read the some introduction in wiki https://en.wikipedia.org/wiki/Kapitsa–Dirac_effect, and it states that the effect was first observed in diffraction of electrons from a standing wave of light. I understand most of the statements in the wiki page. But in other materials, they refer to the similar stuffs with "resonant standing wave" of light. This is confusing to me on the term "resonant". To my understanding, in the text of fundamental physics, standing wave is associated with resonance because it will be formed when the frequency of the "input oscillation" satisfied to some condition. But in other reference on Kapitsa–Dirac effect or in the above wiki page, I saw a term "near resonant standing wave laser field". I don't understand why it said "near resonant", what does it really mean? Thanks.