Strictly speaking, any of a class of plants or drugs which have the effect of modifying sensory input to the brain in ways not related to objective reality. Thus, colors can become brighter, walls may become plastic, etc. Commonly (and incorrectly) equated to substances that cause hallucinations, where the mind creates sensory input rather than modifies it.
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...