Anything that's preservative keeps things alive, healthy, or fresh. Some cooks swear by the preservative powers of lemon juice, which can keep fruit tasting fresh and save avocado slices from turning brown.
Use preservative as an adjective — or a noun, for a chemical or other substance that keeps something from dying or breaking down. Food preservatives are sometimes added to make edible things last longer, and to items like medication and cosmetics, so they don't decay before you've finished using them. Preservative followed the verb preserve, which in the fourteenth century meant "to keep something safe."