Pronunciation (UK) IPA (key): /ˈnjuː.kli.əs/ (US) IPA (key): /ˈnuːkli.əs/ Audio (US)(file) Noun Singular nucleus Plural nuclei or nucleuses (countable) A nucleus is the central part of something, such as a cell or an atom. Our cells store their genetic information in their nucleus. The nucleus of an atom is made up of protons and neutrons.Wikiwand - on Seamless Wikipedia browsing. On steroids.Remove ads