Advection is transport of a substance by a fluid motion.
Usually, advection refers to the distribution, diffusion of some contaminant in the air of in water.
Considering advection, the contaminant is supposed not to affect the movement of the fluid.
http://www.geosci-model-dev-discuss.net/8/2905/2015/gmdd-8-2905-2015.html M.Sofiev, J.Vira, R.Kouznetsov, M.Prank, J.Soares, E.Genikhovich. Construction of an Eulerian atmospheric dispersion model based on the advection algorithm of M. Galperin: dynamic cores v.4 and 5 of SILAM v.5.5. Geosci. Model Dev. Discuss., 8, 2905-2947, 2015.
http://en.wikipedia.org/wiki/Advection In physics, engineering, and earth sciences, advection is a transport mechanism of a substance or conserved property by a fluid due to the fluid's bulk motion.