nucleus

Short definition from Wiktionary

Noun

1. 1
   The core, central part of something, around which other elements are assembled.
2. 2
   An initial part or version that will receive additions.
3. 3
   The massive, positively charged central part of an atom, made up of protons and neutrons.
4. 4
   A large membrane-enclosed organelle found in eukaryotic cells which contains genetic material.
5. 5
   A ganglion, cluster of many neuronal bodies where synapsing occurs.
6. 6
   The central part of a syllable, most commonly a vowel.