septum

from The American Heritage® Dictionary of the English Language, 5th Edition.

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

from The Century Dictionary.

• noun A partition; a wall separating two cavities.
• noun Specifically
• noun In botany, any kind of a partition, whether a proper dissepiment or not: as, the septum in a seed; the septum of a spore.
• noun 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.
• noun 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.
• noun In conchology, one of the transverse partitions which separate the cavity of the shell of a cephalopod into chambers.
• noun In Vermes, a sort of diaphragm, a series of which may partition a worm into several cavities.
• noun In Protozoa, the wall between any two compartments of the test, as of a foraminifer.
• noun Same as septum lucidum.

from the GNU version of the Collaborative International Dictionary of English.

• noun A wall separating two cavities; a partition.
• noun (Bot.) A partition that separates the cells of a fruit.
• noun One of the radial calcareous plates of a coral.
• noun One of the transverse partitions dividing the shell of a mollusk, or of a rhizopod, into several chambers. See Illust. under Nautilus.
• noun One of the transverse partitions dividing the body cavity of an annelid.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun anatomy A wall separating two cavities; a partition; as, the nasal septum.
• noun botany A partition that separates the cells of a fruit.
• noun zoology One of the radial calcareous plates of a coral.
• noun zoology One of the transverse partitions dividing the shell of a mollusk, or of a rhizopod, into several chambers.
• noun zoology One of the transverse partitions dividing the body cavity of an annelid.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (anatomy) a dividing partition between two tissues or cavities
• noun a partition or wall especially in an ovary