An Etymological Dictionary of Astronomy and Astrophysics

1. Core of an atom, where most mass and all positive charge is concentrated. It consists of protons and neutrons.
   
2. Of a comet, the stellar-appearing frozen core, containing most of the cometary mass, in the head of a comet.

Etymology (EN): From L. nucleus “kernel,” from nucula “little nut,” diminutive of nux “nut,” from PIE *knu(k) “lump” (cf. M.Ir. cnu, Welsh cneuen, M.Bret. knoen “nut,” O.N. hnot, O.E. hnutu “nut”).

Etymology (PE): Hasté, variants asté “kernel, fruit stone,” ostoxân “bone,”
from Mid.Pers. astak “fruit stone, bone,” ast “bone;” Av. ast- “bone;” cf. Skt. asthi- “bone;” Gk. osteon; L. os; Hittite hashtai-; PIE base *os-.