What is Nucleus ?
Nucleus is (noun) 1. the central part of an atom, formed of neutrons and protons Electrons orbit the nucleus of the atom. 2. the central body in a cell, containing DNA and RNA, and controlling the way that the cell works First the nucleus divides, then the whole cell splits in two. 3. a centre around which something gathers The six experienced players form the nucleus of the new team. (NOTE: The plural is nuclei .)
source: Easier English, Student Dictionary Upper Intermediate Level