In physics, the term 'nucleus' refers to the tiny, extremely dense core of an atom, where the protons and neutrons are located and bound together by the strong nuclear force. (BTW: does the weak nuclear force play a role in nucleon bonding?)

The nucleus contains practically all of the mass of the atom, the orbiting electrons weighing only 0.00054 (approx.) of the proton or neutron rest mass each. The number of protons in the nucleus (Z) determine the number of electrons in the atom, and thus the chemical properties of the atom. The number of electrons determines the radius of the atom (together with the proton charge], and the number of nucleons determines how massive each atom will be. The number of neutrons determines the stability of the nucleus, and which of a number of isotopes of an element that atom is a member of.

In biology, the term 'nucleus' refers to the all-important organelle in the cell which contains the genetic material, associated proteins and enzymes to transcribe, repair and duplicate this material, and a sub-organelle termed the nucleolus which contains genetic material relating to the ribosomes of the cell. The nucleus is often referred to as the brains of the cell, as it controls and directs the cell's operations and processes by releasing RNA containing instructions for the synthesis of proteins into the cytoplasm to direct the ribosomes in protein synthesis.