Eucalyptus is one of three similar genera that are commonly referred to as "eucalypts," the others being Corymbia and Angophora. Many, but far from all, are known as gum trees because many species exude copious sap from any break in the bark (e.g. Scribbly Gum). The generic name is derived from the Greek words ευ (eu) "well" and καλυπτος (kalyptos) "covered," referring to the operculum on the calyx that initially conceals the flower.