Sillage
Sillage (pronounced see-yazh, from the French word for "wake" — the trail a boat leaves on water) is the scent trail a fragrance leaves in the air around you. It's how noticeable a perfume is to people nearby, how far the scent radiates from the skin, and how strongly that trail persists as you move through a space.
Sillage is one of three properties perfume reviewers use to describe how a fragrance performs in the world, alongside projection (how far the scent extends in front of you, measured in arm's length or steps) and longevity (how many hours the scent lasts on skin). A fragrance with heavy sillage announces itself across a room — think classic Tom Ford Black Orchid or YSL Opium. A fragrance with intimate sillage stays close to the wearer and is only noticed within hugging distance.
How a fragrance achieves its sillage comes down to the perfumer's choice of materials: dense base notes like oud, amber, civet, and patchouli tend to radiate broadly; clean citrus and aquatic notes typically stay close. Concentration matters too — an Extrait de Parfum will almost always project more sillage than the same scent as an Eau de Toilette. Skin chemistry, application method, and even weather (heat amplifies sillage) all shape the trail a fragrance leaves.