Effluvium \ih-FLOO-vee-uhm\ , noun;
1. A slight or invisible exhalation or vapor, especially one that is disagreeable
Effluvium dates to the 1640's and comes from Latin effluvium ("a flowing out") which derives from effluere. The plural is effluvia.
Today's word and the first definition were both taken from Dictionary.com's 'Word of the Day' for Friday, February 4
Etymologies come from the Oxford English Dictionary and/or Etymonline.com