What is the meaning of nucleus?

noun

• A part of the cell containing DNA and RNA and responsible for growth and reproduction.
  • synonyms: cell nucleus, karyon
• The positively charged dense center of an atom.
• A small group of indispensable persons or things.
  • usage: "five periodicals make up the core of their publishing program"
  • synonyms: core, core group
• 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. (
• Any histologically identifiable mass of neural cell bodies in the brain or spinal cord.
• The central structure of the lens that is surrounded by the cortex.
  • synonyms: lens nucleus