Smell, or olfaction, is the sense that detects airborne chemical molecules. It contributes to the flavor of food, warns of danger, and evokes powerful emotional and autobiographical memories.

Odorants bind to receptors on cilia of olfactory receptor neurons high in the nasal cavity. Humans possess several hundred different receptor types; each odor is encoded by a pattern of activation across the receptor population. Signals travel via the olfactory nerve to the olfactory bulb and then to cortical and limbic areas, including regions involved in emotion and memory. Unlike other senses, olfaction has a relatively direct route to these structures, which helps explain the vividness of odor-evoked recollection.

Sensitivity varies widely among individuals and declines with age and certain diseases. Adaptation to constant odors is rapid. Cultural experience and learning shape which smells are judged pleasant or unpleasant. Although human olfaction is often underestimated, it remains acute enough to support discrimination of an enormous number of odor qualities.

A single molecule in the air can trigger recognition, emotion, and memory all at once.