The Collaborative International Dictionary of English v.0.48:

Effluvium \Ef*flu"vi*um\, n.; pl. Effluvia. [L., a flowing
   out, fr. effluere to flow out. See Effluent, a.]
   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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

effluvium
    n 1: a foul-smelling outflow or vapor (especially a gaseous
         waste)