A subtle outflow, an exhaled vapor, scent, or intangible emanation. Often used for odors that cling in the air like a memory. Something has come off a body or place and now haunts the space around it.

From Latin effluvium “a flowing out,” from effluere “to flow out” (ex- “out” + fluere “to flow”). The word began as a theory of emanations and ended as a perfectly practical way to name what lingers.