In Europe and among some Pagans, any cereal grains such as wheat, barley, rye, etc., but excluding maize. Curiously, in the US it refers to maize and excludes other grains.
Readers, please enjoy this guest blog post by Sandra Kynes, author of several books, including The Herb Gardener's Essential Guide, Plant Magic, and the new Witches' Encyclopedia of Magical Plants, among many others.
Although a few old books...