The Collaborative International Dictionary of English v.0.48:
Champignon \Cham*pi"gnon\, n. [F., a mushroom, ultimately fr. L. campus field. See Camp.] (Bot.) An edible species of mushroom (Agaricus campestris). [1913 Webster] Fairy ring champignon, the Marasmius oreades, which has a strong flavor but is edible. [1913 Webster]