- For the abbreviation SEPTA, see SEPTA.
A septum, in general, is a wall separating two cavities or two spaces containing a less dense material. The word is Latin and literally means "something that encloses". (plural: septa)
The term is most commonly used in anatomy and biology:
- the septum nasi, or simply septum, is the cartilage wall separating the two nostrils
- the interventricular septum (or median septum) is the wall separating the two halves of the heart
- the septum pellucidum, or septum lucidum, is a thin structure separating two fluid pockets in the brain
- a septum of the uterus (uterine malformation)
- in histology, microscopic fibrous septa are seen throughout most tissues of the body, particularly where they are needed to stiffen a soft cellular tissue, and they also provide planes of ingress for small blood vessels. Because the dense collagen fibres of a septum usually extend out into the softer adjacent tissues, histological septa are less clearly defined than the macroscopic types of septa listed above. Sometimes septum is a cross-wall.
- in marine biology the thin membrane separating each siphuncle, or chamber, in the shells of nautiloids, ammonites, and belemnites—all cephalopods that retained their external shell.
The term septum is also used to refer to man-made dividers, such as those used to close vials of injectable fluid. These may be designed to be pierced by a hypodermic needle.
A septum may also be a device used with a circular particle beam accelerator to inject or eject a beam of particles to or from the accelerator. Septa can deflect an ejected beam while not affecting the orbiting beam. The two types of septum are the electrostatic septum and the septum magnet.