What does basion mean?
The basion is a precisely defined anatomical landmark on the human skull: it is the midpoint of the anterior margin of the foramen magnum, the large opening at the base of the occipital bone through which the spinal cord passes into the braincase. Because this point is stable and easily identified on lateral X-rays and CT scans, it serves as a fundamental reference in craniometry, the measurement of skulls, and in modern cephalometric analysis used by radiologists, neurosurgeons, and orthodontists. Measurements taken relative to the basion help clinicians evaluate conditions such as basilar invagination, in which part of the spine protrudes into the skull, and assist anthropologists in comparing cranial morphology across populations. The word derives from the Greek basis, meaning 'base' or 'foundation', reflecting its position at the very floor of the skull. Though obscure outside specialist fields, the basion is indispensable wherever accurate description of skull-base anatomy is required.
nounIn anatomy and craniometry, the midpoint of the anterior margin of the foramen magnum at the base of the skull; it serves as a standard cephalometric landmark.
- The midpoint of the anterior margin of the foramen magnum, used as a cephalometric and craniometric landmark.
"The radiologist measured the angle formed by the nasion, sella, and basion to assess the patient's skull base."
"On the lateral cephalogram, the technician traced the outline of the foramen magnum and marked the basion with precision."
"Displacement of the odontoid process above the basion is one sign used to diagnose basilar invagination."
Rarely pluralized, since each skull has only one basion; the plural appears mainly when referring to the landmark across multiple specimens.
"The study recorded the basions of two hundred skulls from the museum's collection."
Every human skull has a fixed reference point called the basion — the exact spot where your spinal cord enters your braincase — and craniometrists have used it as an anchor for measuring heads since the nineteenth century.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of basion
The term basion comes from the Greek basis, meaning 'base' or 'foundation', combined with the adjectival ending -on. It was adopted into scientific Latin and then English during the nineteenth century, when craniometry was being formalized as a discipline and standardized landmarks on the skull were given precise names. It shares its root with related anatomical terms such as 'basilar', which likewise refers to structures at the base of the skull.
How basion is actually used
A technical term confined to anatomy, physical anthropology, forensic science, and radiology; it is rarely encountered outside specialist literature. It is always used in its singular form when referring to the landmark itself.
Easily confused with basion
The nasion is the landmark at the bridge of the nose where the frontal and nasal bones meet, while the basion is a landmark on the anterior rim of the foramen magnum at the skull base.