If your parents treated your cold with Vick's Vaporub when you were a child, you know the powerful and pungent scent of Eucalyptus Leaf, which also goes by the scientific name Eucalyptus globulus and colloquially as the "blue gum tree." Although all species of Eucalyptus are native to Australia, not all have been successfully cultivated outside of that country; Eucalyptus globulus however is the most widely cultivated species outside of Australia, and virtually all dried Eucalyptus Leaf comes from this species. (It has in fact become an invasive species in California.)
Eucalyptus Leaf is just that, the leaf of a tree rather than an herb in the technical sense, but traditionally, people of Asia, Australia and Oceania have found it no less useful. Fresh c/s Eucalyptus leaf is steamed in order to extract the essential oil; the strong scent of the oil extracted from this natural herb is claimed to be therapeutic and is pleasing to some. However, it is not pleasing to insect pests.
Most commonly, dried or powdered Eucalyptus Leaf is brewed up as an herbal tea.