Meaning

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

noun the positively charged dense center of an atom

noun a small group of indispensable persons or things

originEtymology tree Proto-Italic *knuks Latin nux Latin nucleuslbor. English nucleus Learned borrowing from Latin nucleus (“kernel, core”). The earliest uses refer to the head of a comet and the kernel of a seed, both recorded in Lexicon Technicum in 1704. The sense in atomic physics was coined by British scientist Michael Faraday in 1844 in a theoretical meaning.

Etymology from Wiktionary, CC BY-SA 3.0