n a part of the cell containing DNA and RNA and responsible for growth and reproduction n the positively charged dense center of an atom n a small group of indispensable persons or things n (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 n any histologically identifiable mass of neural cell bodies in the brain or spinal cord n the central structure of the lens that is surrounded by the cortex