A nucleus is the control center of a living cell, a tiny sphere that contains all the instructions needed to build and run that cell. Think of it as the cell's library and command center combined: it holds the DNA that determines everything from your eye color to how tall you'll grow.

The nucleus sits inside most cells in your body, surrounded by its own protective membrane. Inside, the DNA is organized into structures called chromosomes, which contain genes: the specific instructions for making proteins and controlling what the cell does. When a cell needs to make a particular protein, it reads the instructions from the nucleus, like checking out a book from a library.

Not all cells have a nucleus. Bacteria are prokaryotic cells, meaning they lack a true nucleus and instead have their DNA floating freely inside the cell. But your cells, along with those of all animals, plants, and fungi, are eukaryotic, meaning they have a proper nucleus keeping the DNA organized and protected.

The word also appears in physics, where the nucleus of an atom is the dense center made of protons and neutrons. Scientists also use nucleus to describe any central, essential part around which other things gather, like calling a small group of dedicated volunteers the nucleus of a larger organization.