nucleus

noun (plural nuclei /-lɪʌɪ/)

• the central and most important part of an object, movement, or group, forming the basis for its activity and growth:the nucleus of a British film-producing industry
• Physics the positively charged central core of an atom, consisting of protons and neutrons and containing nearly all its mass.
• Biology a dense organelle present in most eukaryotic cells, typically a single rounded structure bounded by a double membrane, containing the genetic material.
• Anatomy a discrete mass of grey matter in the central nervous system.
• Astronomy the solid part of a comet’s head.