The story
Named after Hungarian-American physicist Eugene Wigner, who described the experiment in an article published in 1961.
Etymology from Wiktionary, CC BY-SA 4.0.
Wigner's friend
Today it means: The hypothetical observer in a thought experiment in which Wigner, absent when his friend observes the state of a particle (in some versions, Schrödinger's cat) as it collapses from quantum superposition, concludes that both the particle and his friend remain in quantum superposition until Wigner himself learns the result of the observation. Full definition →