n, pl  nu*clei  also  nu*cle*us*es  1  :  a cellular organelle of eukaryotes that is essential to cell functions (as reproduction and protein synthesis), is composed of nuclear sap and a nucleoprotein-rich network from which chromosomes and nucleoli arise, and is enclosed in a definite membrane  2  :  a mass of gray matter or group of nerve cells in the central nervous system  3  :  a characteristic and stable complex of atoms or groups in a molecule : esp  :  RING 2 <the naphthalene >   4  :  the positively charged central portion of an atom that comprises nearly all of the atomic mass and that consists of protons and neutrons except in hydrogen which consists of one proton only .
 
 
 
Similar sounding terms:  nu·cle·ase 
 
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