Effluvium (plural, effluvia)
1. A, usually invisible, emanation or exhalation of vapour, gas or small particles.
2. A by-product or residue; waste.
3. The odorous fumes given off by waste or decaying matter.
4. An impalpable emanation; an aura.

From the Latin ex out + fluĕre to flow.
The OED records the first English usage in 1646.