noun
Definition 1
Positively charged central portion constituting main mass of atom.
Source: Concise Oxford Dictionary
Context: She [Austrian Lise Meitner] showed that the nucleus of the uranium atom is so unstable that it breaks in two if hit by another atomic particle.
Source: Financial Times 28/09/2009 (review by Alan Cane of "Uranium Wars: The Scientific Rivalry that Created the Nuclear Age" by Amir D Aczel
Definition 2
Central part or thing around which others are gathered.
Source: Concise Oxford Dictionary
Context: The title [of the play "Justifying War"] may refer to the invasion of Iraq or to the battle between the BBC and Downing Street, but the production rightly emphasizes that the nucleus of the affair is a private tragedy.
Source: independent.co.uk. 06/11/2003 (review by Paul Taylor)