Pronounced "See-ah-j", is the French/technical term for the trail created by a scent when it is worn on the skin. It comes from the French for "wake" and can best be described as how a fragrance diffuses "in a person's wake," or behind the wearer as they move. A fragrance need not be a heavy one to have a large sillage.
Sillage is not the same as projection (how a fragrance is perceived by others around the wearer). Instead, it is enhanced by motion and ambient temperature as well as the inherent qualities of the skin (as well as, to refer to my last post, if it contains Davana flower, or other similar "chameleon" ingredients). It (the skin chemistry) becomes a substrate to the scent.
For the aforementioned "ambient temperature", this is why some scents, such as the Rayhaan Lion eau de parfum I previously posted, would be considered "too intense" for summer/hot weather. Whereas, the Armani Stronger With You line, for example, is better in cold weather; claimed by many to have a sillage over a kilometer.
The video attached to this features a perfume student rating different men's colognes, and also mentions "sillage", among other terms. Also, her reaction to Armani SWYA from 4:58 to 5:30 is priceless ;-)