nucleus [M17th] A nucleus is literally a ‘little nut’, Latin nucleus or nuculeus being formed from nuci-, nux, ‘a nut’, and used to mean ‘the inner part or kernel’. In English, it was originally used for the bright core forming the head of a comet, then transferred to a number of different objects with the sense of inner core, the use in biology being mid 19th century. Its first use in physics was mid 19th century in atomic theory, becoming the positively charged central core of the atom in the early 20th century. Nuclear is found from the mid 19th century referring to comets and to the nucleus of a cell, and from the early 20th century in physics. Those who pronounce the word as if it spelt nucular may find comfort in the fact that the two Latin forms suggest a similar problem.