a colourless liquid abundant in the essential oils of pine and citrus trees and used as a lemonlike odorant in industrial and household products and as a chemical intermediate.

Limonene exists in two isomeric forms (compounds with the same molecular formula—in this case, C₁₀H₁₆—but with different structures), namely l-limonene, the isomer that rotates the plane of polarized light counterclockwise, and d-limonene, the isomer that causes rotation in the opposite direction. In the extraction of citrus juices d-limonene is obtained as a by-product, and it also occurs in caraway oil; l-limonene is present in pine needles and cones; dl-limonene, or dipentene, the mixture of equal amounts of the l- and d-isomers, is a component of turpentine.

Dipentene may be sulfurized to produce additives that improve the performance of lubricating oils under heavy loads; d-limonene is commercially converted to l-carvone, which has a caraway-seed flavour.