nucleus

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

• noun A central or essential part around which other parts are gathered or grouped; a core.
• noun Something regarded as a basis for future development and growth; a kernel.
• noun Biology A membrane-bound organelle within a eukaryotic cell that contains most of the cell's genetic material. DNA transcription takes place in the nucleus.
• noun Anatomy A group of specialized nerve cells or a localized mass of gray matter in the brain or spinal cord.
• noun Physics The positively charged central region of an atom, composed of protons and neutrons, about which negatively charged electrons orbit. Extremely small and dense, the nucleus contains almost all of the mass of an atom.
• noun Chemistry A group of atoms bound in a structure, such as a benzene ring, that is resistant to alteration in chemical reactions.
• noun The central, often brightest part of the head of a comet.
• noun The solid part of a comet, composed of ice and smaller amounts of dust and rock.
• noun The central, often brightest part of a galaxy.
• noun Meteorology A particle on which water vapor molecules accumulate in free air to form a droplet or ice crystal.
• noun Linguistics The part of a syllable having the greatest sonority. In the word middlemost (mĭd′l-mōst″) the nuclei of the three syllables are (ĭ), (l), and (ō); in the Czech word krk (“neck”), the nucleus is (r).

from The Century Dictionary.

• noun A kernel; hence, a central mass about which matter is collected, or to which accretion is made; any body or thing that serves as a center of aggregation or assemblage; figuratively, something existing as an initial or focal point or aggregate: as, a nucleus of truth; a nucleus of civilization.
• noun In biology, the kernel of a cell, in general; a central or interior differentiated mass of protoplasm, found in nearly all cells, vegetable or animal, and consisting of an oval or rounded body composed of
• noun a nuclear membrane
• noun nuclear network, and
• noun nucleoplasm, and containing nucleoli. The nuclear network is made up of threads or fibrils which are composed of a deeply staining part, “chromatin,” and a feebly staining intermediate substance, “linin” or parachromatin (nucleohyaloplasm). In the meshes of the network is found the more fluid part of the nucleus, the nucleoplasm (achromatin, karyochylema, paralinin). Nucleoplasm, according to Carnoy, consists of a plastin network and a granular fluid, “enchylema.” The nuclear membrane is considered by some observers to be an inner limiting layer of cell-protoplasm surrounding the nucleus, by others to be a condensation of the peripheral portion of the nuclear network. There may be but one nucleus or several nuclei in one cell; and a nucleus may be nucleolate or not. Nuclei are generally proportionate in size to the cell containing them: in some instances, however, they form almost the entire cell mass. A structural difference between the nucleus and the rest of the cell-protoplasm is indicated by its greater resistance to powerful reagents, and by its varied reaction with stains. Functionally, the nucleus is the most important portion of the cell, as it is here that the complex series of changes known as karyokinesis take place, resulting in the division of the nucleus and followed by the division of the cell. This process of mitosic or indirect cell-division is found in all varieties of cells, whether vegetable or animal, fetal or adult, normal or pathological. Instances of cell-division not mitosic have, however, been noted. The nucleus of the human ovum was discovered by Purkinje in 1825, and hence is often called the corpuscle of Purkinje. Its usual name in text-books of anatomy is germinal vesicle. See cut under cell, 5.
• noun In zoology:
• noun In ascidians, the alimentary and reproductive viscera collectively, when these are aggregated into a mass, as in the salps.
• noun In protozoans, a solid rod-like or strap-shaped body, having in many cases the functions of an ovary in connection with a nucleolus (see nucleolus, 2).
• noun In echinoderms, the madreporiform body
• noun In anatomy, a collection of ganglion-cells in the brain or other portion of the cerebrospinal axis.
• noun In conchology, the embryonic shell which remains at the apex of the mature shell, as of a gastropod; also, the initial point from which the operculum of a gastropod grows. See protoconch.
• noun A body having a stronger or weaker attraction for the gas, vapor, or salt of a solution than for the liquid part of it, and therefore modifying by its presence the freezing-and boiling-points.
• noun In astronomy, the bright central point usually present in the head of a comet and often in a nebula.
• noun A genus of gastropods: same as Columbella.
• noun Same as claustrum, 1.
• noun A very small colony of honey-bees started for the purpose of rearing queens to exchange with the queens of full colonies, thus assisting in the prevention of swarming.
• noun no points at all;
• noun a perfect set. This result (set) is the nucleus EΩ.

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

• noun A kernel; hence, a central mass or point about which matter is gathered, or to which accretion is made; the central or material portion; -- used both literally and figuratively.
• noun (Astron.) The body or the head of a comet.
• noun An incipient ovule of soft cellular tissue.
• noun A whole seed, as contained within the seed coats.
• noun (Biol.) A body, usually spheroidal, in a eukaryotic cell, distinguished from the surrounding protoplasm by a difference in refrangibility and in behavior towards chemical reagents, which contains the chromosomal genetic material, including the chromosomal DNA. It is more or less protoplasmic, and consists of a clear fluid (achromatin) through which extends a network of fibers (chromatin) in which may be suspended a second rounded body, the nucleolus (see Nucleoplasm). See Cell division, under Division.
• noun The tip, or earliest part, of a univalve or bivalve shell.
• noun The central part around which additional growths are added, as of an operculum.
• noun A visceral mass, containing the stomach and other organs, in Tunicata and some mollusks.

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

• noun The core, central part (of something), round which others are assembled.
• noun An initial part or version that will receive additions.
• noun chemistry, physics The massive, positively charged central part of an atom, made up of protons and neutrons.
• noun cytology A large organelle found in cells which contains genetic material.
• noun neuroanatomy A ganglion, cluster of many neuronal bodies where synapsing occurs.
• noun linguistics The central part of a syllable, most commonly a vowel.

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

• noun the positively charged dense center of an atom
• noun any histologically identifiable mass of neural cell bodies in the brain or spinal cord
• noun (astronomy) the center of the head of a comet; consists of small solid particles of ice and frozen gas that vaporizes on approaching the sun to form the coma and tail