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.

a foul-smelling outflow or vapor (especially a gaseous waste)

shedding, especially of hair.

effluvium, a flowing out], an outflow, which may pertain to sudden, severe hair shedding

Effluvium refers to an outflowing or shedding, or exhalation or emanation, particularly one that is noxious or toxic.