osteon

right|371x371px|thumb|Diagram of a typical long bone showing both compact (cortical) and cancellous (spongy) bone. thumb|Osteons on cross-section of a bone

In osteology, the osteon or haversian system (; named for Clopton Havers) is the fundamental functional unit of much compact bone. Osteons are roughly cylindrical structures that are typically between 0.25 mm and 0.35 mm in diameter. Their length is often hard to define, but estimates vary from several millimeters to around 1 centimeter. They are present in many bones of most mammals and some bird, reptile, and amphibian species.