The most readily recognisable characteristics of Eucalyptus species are its distinctive flowers and fruit (capsule).
Flowers have numerous fluffy stamens, which may be white, cream, yellow, pink or red; in bud the stamens are enclosed in a cap known as an operculum, which is composed of the fused sepals or petals or both. Thus flowers have no petals, decorating themselves instead with the many showy stamens. As the stamens expand, the operculum is forced off, splitting away from the cup-like base of the flower; this is one of the features that that unites the genus. The name Eucalyptus, from the Greek words eu-, well, and kaluptos, cover, meaning "well-covered", describes the operculum.