Main Entry: nu·cle·us

Pronunciation: \ˈn(y)ü-klē-əs\

Function: noun

Inflected Form(s): plural nu·clei \-klē-ˌī\ also nu·cle·us·es

1 : a cellular organelle of eukaryotes that is essential to cell functions (as reproduction and protein synthesis), is composed of nuclear sap and a nucleoprotein-rich network from which chromosomes and nucleoli arise, and is enclosed in a definite membrane

[nucleus illustration]

2 : a mass of gray matter or group of nerve cells in the central nervous system
3 : a characteristic and stable complex of atoms or groups in a molecule ; especially : ring 2 <the naphthalene nucleus>
4 : the positively charged central portion of an atom that comprises nearly all of the atomic mass and that consists of protons and neutrons except in hydrogen which consists of one proton only