Meaning of “nucleus” - Learner’s Dictionary

nucleus

noun [ C ] uk us /ˈnjuːkliəs/ (plural nuclei /ˈnjuːkliaɪ/)
ATOM OR CELL

the central part of an atom or cell

IMPORTANT PART

the central or most important part of a group or idea:

Senior coaches handpicked the nucleus of the team.

(Definition of “nucleus” from the Cambridge Learner’s Dictionary © Cambridge University Press)