Webster's English Dictionary

Enter the word to look up:

nucleus

nu.cle.us \'n(y)u:-kle--*s\ \-kle--.i-\ n or nu.clei also nu.cle.us.es [NL, 
   fr. L, kernel, dim. of nuc-, nux nut - more at NUT] pl  1: the small, 
   brighter, and denser portion of a galaxy or of the head of a  comet 2: a 
   central point, group, or mass about which gathering, concentration, or  
   accretion takes place : as 2a: a portion of cell protoplasm held to be 
   essential to vital phenomena an d heredity, made up of a network rich in 
   nucleoproteins from which chromosomes and nucleoli arise and a hyaline 
   ground substance, and enclosed by a definite membrane 2b: a mass of gray 
   matter or group of nerve cells in the central nervous sy stem 2c: a 
   characteristic and stable complex of atoms or groups in a molecule  2d: 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