nucleus

(noun) a part of the cell containing DNA and RNA and responsible for growth and reproduction. Synonyms: cell nucleus, karyon.

(noun) the positively charged dense center of an atom.

(noun) a small group of indispensable persons or things. Synonyms: core, core group. Examples: "Five periodicals make up the core of their publishing program."

(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.

(noun) any histologically identifiable mass of neural cell bodies in the brain or spinal cord.

(noun) the central structure of the lens that is surrounded by the cortex. Synonyms: lens nucleus.