/
ˈnjuːklɪəs/
n.
 (pl. nuclei  
/-lɪʌɪ/
)
    - the central and most important part of an object or group.
 - Physics  the positively charged central core of an atom, containing nearly all its mass.
 - Biology  a structure present in most cells, containing the genetic material.
 - a discrete mass of grey matter in the central nervous system.
 
Etymology
C18: from L., lit. ‘kernel, inner part’, dimin. of nux, nuc- ‘nut’.