od

od \od, ohd\, noun:

a hypothetical force formerly held to pervade all nature and to manifest itself in magnetism, mesmerism, chemical action, etc.

"I studied mind-cure, or metaphysical healing, which strikes at the root of disease; I went into hypnotism, mesmerism, and phreno-magnetism, and the od force—I don't suppose you know about the od which Reichenbach discovered."
-- Edward Eggleston, The Faith Doctor: a Story of New York, 1891

In these experiments, different substances from which the od was supposed to emanate were arranged on a collodion plate.
-- Edited by Clément Chéroux, The Perfect Medium: Photography and the Occult, 2005

The term od was coined by chemist and philosopher Karl Ludwig von Reichenbach as a name for his hypothetical force. He proposed the name od because he thought a short word starting with a vowel would be more easily combined in compound words.