Is a method of extracting the aromatic compounds and properties out of raw materials. These plant-based, organic materials are put through a specific set of exercises in which the raw materials are placed into a pressurized environment where they are compressed and compacted extensively. This process is undertaken with the specific intention of releasing the fragrant and aromatic essential oils being kept in mind. These resulting essential oils have multiple uses in perfumery as well as various other industries. In modern times, this technique is largely only partaken in order to extract fragrant substances out of the peels of citrus fruits, as the technique can be incredibly wasteful and inefficient when performed on less sturdy materials. Citrus peel also contains vast quantities of oil — being oil rich in nature — and thus makes the process of expression extraction economically viable. The process is easy and quick; yielding great amounts of oil when performed on citrus peel, and the low-cost and relative abundance of citrus materials has made expression somewhat synonymous with the citrus family. Expression can also be performed through either the mechanical or cold-press (hydraulic) techniques.

The process is also inadvertently performed in the commercial citrus juice industry, where the pressing and squeezing of citrus fruits mimics the processes of expression. This unintentional expression extraction leaves behind an abundance of by-products that are useless in citrus juice production, but are incredibly useful to the perfume industry. Much of the oils of the sweet orange, as well as of the lemon, that are seen in perfumery, come via this bi-product phenomenon.