An aroma is defined by the total number of odours attributable to dozens of different volatile compounds. What chemists call an "aroma compound" is an isolated molecule of an aroma.

Aromas are made up of various classes of chemical molecules, including terpenes, lactones, pyrazines, ethers, esters and others. There are terpenes that account for the characteristic odour of essential oils. Lactones are associated with an impression of a fruity odour, like the d-decalactone referred to above, which conveys an odour of peach. Pyrazines are aroma compounds that are characteristic of heated foods, while esters have fruity notes, like ethyl valerate, which is an aroma compound characteristic of apples.