nucleus

Where it comes from

Nucleus was adopted from Latin nucleus, a kernel. That noun was connected with nucula, a little nut, the diminutive of nux, nut. English first used nucleus for a kernel or compact central body before science gave it several specialized centers.

How the meaning developed

The kernel inside a shell supplied a durable picture of a compact center. Astronomers applied nucleus to a comet's bright center, biologists to the body inside a cell, and physicists to the tiny central part of an atom. The scientific objects differ, but the kernel metaphor holds them together.