A eucalyptus () is a plant in the genus Eucalyptus, which consists of more than 700 species of flowering plants in the family Myrtaceae. Most species of eucalyptus are trees, often mallees, and a few are shrubs. Along with several other genera in the tribe Eucalypteae, including Corymbia and Angophora, they are commonly known as eucalypts or "gum trees". Eucalyptuses have bark that is either smooth, fibrous, hard, or stringy and leaves that have oil glands. The sepals and petals are fused to form a "cap" or operculum over the stamens, hence the name from Greek eû ("well") and kaluptós ("covered"). The fruit is a woody capsule commonly referred to as a "gumnut".
Most species of eucalyptus are native to Australia, and every state and territory has representative species. About three-quarters of Australian forests are eucalypt forests. Many eucalypt species have adapted to wildfire, are able to resprout after fire, or have seeds that survive fire.