Definition of "Nucleus" :

noun: the central structure of the lens that is surrounded by the cortex

noun: a part of the cell containing DNA and RNA and responsible for growth and reproduction

noun: any histologically identifiable mass of neural cell bodies in the brain or spinal cord

noun: a small group of indispensable persons or things

noun: the positively charged dense center of an atom

noun: (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