Have you ever been alone on an elevator, but judging by a lingering smell of cologne or perfume, it's quite apparent that someone else was just there? Like a snail that leaves a trail behind, perfume can also leave a trail behind, too. It's called sillage, which is French for "wake," as in the stream left behind a boat.
It's a term used by perfume enthusiasts to describe how close a fragrance stays to the skin. Does it stick to the body or does it diffuse, leaving behind a wafting aroma? So the next time you're sitting next to a pleasantly perfumed lady on the bus, perhaps she was light on the sprays, or perhaps it's that her perfume has minimal sillage.