What is subtile or invisible emanation; exhalation perceived by the sense of smell; especially, noisome or noxious exhalation; as, the effluvium from diseased or putrefying bodies, or from ill drainage called?

effluvium — Subtile or invisible emanation; exhalation perceived by the sense of smell; especially, noisome or noxious exhalation; as, the effluvium from diseased or putrefying bodies, or from ill drainage. (Webster's 1913.)