Dictionary.net

A name given to the imponderable fluid to which the magnet owes its virtues. By analogy it is applied to a particular principle, supposed to be the source of organic actions, which affects, it is conceived, the nervous system principally, and is susceptible of being transmitted from one living body to another, by contact or simple approximation, and especially under the influence of fixed volition. See Magnetism, Animal.

A supposititious imponderable f. by whose agency the phenomena of magnetism manifest themselves.