GHOST, n. The outward and visible sign of an inward fear.

Ambrose Bierce (1842-1914?) American writer and journalist

“Ghost,” The Cynic’s Word Book (1906)
    (Source)

A play on the traditional Christian (from St Augustine) definition of sacrament: "an outward and visible sign of inward and spiritual grace."

Included in The Devil's Dictionary (1911). Originally published in the "Devil's Dictionary" column in the San Francisco Wasp (1885-02-28).