A septum, in general, is a wall separating two cavities. 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 is the wall separating the two halves of the heart
- the septum is also a dividing structure in the brain
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.