Lavender is a Mediterranean shrub of the genus Lavandula, and its name records its oldest use: it derives from the Latin lavare, to wash, from the Roman practice of scenting baths and laundry with it. Its clean, calming reputation is bound up with its chemistry, the flowers are rich in linalool and linalyl acetate, the compounds responsible for the scent, and it was a standard strewing herb, laid on floors and packed among linen to freshen rooms and repel moths and insects. That much is straightforward and observable: lavender smells strongly and pleasantly, and insects dislike it.
Lavender had a genuine practical role in the First World War: with conventional antiseptics in short supply, lavender oil was used in some British and French field hospitals to dress wounds, a use documented in period nursing accounts and grounded in the oil's real antimicrobial properties rather than in any older folk claim; the practice faded once manufactured antiseptics and antibiotics became reliably available. A related species distinction matters commercially: true lavender, Lavandula angustifolia, is prized for its fine, sweet scent, while lavandin, Lavandula x intermedia, a natural hybrid with a coarser, camphorous note, yields far more oil per plant and supplies most of the lavender used in soap and household products today.