Eucalyptus is a diverse genus of flowering trees and shrubs (including a distinct group with a multiple-stem mallee growth habit) in the myrtle family, Myrtaceae. Members of the genus dominate the tree flora of Australia. There are more than 700 species of eucalyptus, mostly native to Australia, and a very small number are found in adjacent areas of New Guinea and Indonesia and one, Eucalyptus deglupta, ranges north to the Philippines. Only fifteen species occur outside Australia, with just nine of these not occurring in Australia. Species of eucalyptus are cultivated widely in the tropical and temperate world, including the Americas, Europe, Africa, the Mediterranean Basin, the Middle East, China and the Indian Subcontinent, though most species do not tolerate frost.
On warm days eucalyptus forests are sometimes shrouded in a smog-like mist of vaporised volatile organic compounds (terpenoids); the Australian Blue Mountains take their name from the haze.