a small bright and dense part of a galaxy or head of a comet
a central point, mass, etc about which gathering, concentration, etc takes place: e.g
a usu round organelle (specialized structure within a cell) containing the chromosomes and surrounded by a membrane
a discrete mass of nerve cells in the brain or spinal cord
the positively charged central part of an atom that accounts for nearly all of the atomic mass and consists of protons and usu neutrons
[Latin nucleus kernel, dimin. of nuc-, nux nut]