noun pl. nuclei -·clei′ (-ī′) or nucleuses -·cle·uses

1. a thing or part forming the center around which other things or parts are grouped or collected; core
2. anything serving as a center of growth or development the nucleus of a library
3. Anat. a group of nerve cells in the brain or spinal column
4. Astron. the bright central part of the head of a comet
5. Biol. the central, usually spherical or oval mass of protoplasm present in most plant and animal cells, containing most of the hereditary material and necessary to such functions as growth, reproduction, etc.
6. Bot. the central point in a starch grain
7. Chem., Physics the central part of an atom, the fundamental particles of which are the proton and neutron, except for hydrogen, which is usually composed of one proton only: it carries a positive charge and constitutes almost all of the mass of the atom
8. Phonet. the most sonorous portion of a syllable, usually a vowel
9. Organic Chem. a fundamental, stable arrangement of atoms that may occur in many compounds by atomic substitution without structural change, as the benzene ring

Etymology: ModL < L, a nut, kernel, for nuculeus, dim. < nux (gen. nucis), nut