septum

A thin partition or membrane that divides two cavities or soft masses of tissue in an organism.

A partition; a wall separating two cavities.

Specifically

In botany, any kind of a partition, whether a proper dissepiment or not: as, the septum in a seed; the septum of a spore.

In anatomy and zoology, a partition; a wall between two cavities, or a structure which divides a part or an organ into separate portions; a dissepiment. In vertebrates the formations known as septa are most frequently situated in the vertical longitudinal median line of the body, but may be transverse or otherwise disposed. A number of them are specified by qualifying words. See phrases following.

In corals, a calcified mesentery; one of the six or more vertical plates which converge from the wall to the axis of the visceral space, dividing this into a number of radiating loculi or compartments. Each septum appears single or simple, but is really a duplicature of closely apposed plates, just as the mesentery itself is a fold. They are to be distinguished from the horizontal dissepiments, or tabulæ, which may cut them at right angles. They are variously modified in details of form, may be connected by synapticlæ, and are divided, according to their formation, into primary, secondary, and tertiary.

In conchology, one of the transverse partitions which separate the cavity of the shell of a cephalopod into chambers.

In Vermes, a sort of diaphragm, a series of which may partition a worm into several cavities.

In Protozoa, the wall between any two compartments of the test, as of a foraminifer.

Same as septum lucidum.