Volatile oils
The light, easily evaporating molecules that carry most of the scent of spices and fade fastest.
Also: aroma compounds, terpenes, volatiles, aromatics
Volatile oils are small molecules that evaporate readily at room temperature. They reach the nose, both directly and from the back of the mouth, and carry most of what we call spice aroma. Heat, air, light and grinding all speed their loss, which explains why a freshly cracked cardamom pod smells so much richer than stale powder. Cooks protect them by adding fragile spices late, grinding at the last moment and storing in the dark. Others, such as pepper's piperine or turmeric's curcumin, are non-volatile and stay put. A balanced flavour takes both kinds: volatile oils for aroma, non-volatile compounds for bite and colour.