Terpene
The volatile aromatic compounds a cannabis plant produces in its trichomes alongside cannabinoids. They account for how a cultivar smells and tastes, and they evaporate readily.
Terpenes are built from repeating five-carbon isoprene units and are found throughout the plant kingdom, where they commonly serve as attractants or deterrents. In cannabis they are produced in the same glandular trichomes that produce cannabinoids.
A cultivar's aroma is a blend rather than a single compound. Myrcene, limonene, caryophyllene, pinene, linalool, humulene and terpinolene are the ones most often reported on a certificate of analysis, and each has its own page in the terpene index.
They are volatile by definition, which is the practical problem with them. Heat, time, agitation and low humidity all drive them off, so a share of what the plant produced at harvest is gone by the time flower reaches a customer.
Because they evaporate, terpene retention is mostly a question of drying, curing, handling and storage rather than of what happens in the flower room. Claims that a particular late-flower input preserves terpenes are common, and most of them have not been tested in published work.