Nucleus

Latin for center or core.

1. In chemistry and physics, the term used to describe that collection of protons and neutrons in the center of an atom that carry the bulk of the atoms mass and positive charge.
2. In biology, nucleus refers to the membrane-bound subcellular organelle found in eukaryotes, visible via microscopy, which contains, primarily, the cell's chromosomes. See cell nucleus.
3. In astronomy, the solid core of a comet is referred to as the comet's nucleus.