means The dense central core of something — the protons-and-neutrons heart of an atom, the DNA-keeping center of a cell, or the small foundational group around which a larger thing organizes.

from From Latin nucleus, meaning 'kernel' or 'little nut' — a diminutive of nux, 'nut.' So the word's deep image is a tiny seed at the center of a shell, the hard pip you bite down on. Science borrowed it in the 17th century for cell cores and later for the atom's core, keeping that same picture: the small hard center everything else wraps around. The Latin nux is also a distant cousin of English 'nut' itself.