An atomic nucleus and a cell nucleus are very different! However, the word is the same because they share a very broad similarity. When used generally, the word "nucleus" refers to something at the center of something else. Since both atomic nuclei and cell nuclei are at the center of atoms and cells, they are both called the same thing: a nucleus.

An atomic nucleus is the region at the center of an atom, made up of protons and neutrons. Relative to the size of the atom, the nucleus is tiny, over 10,000 times smaller than the total size of the atom, which includes the electrons. A cell nucleus is the region at the center of a cell that contains all the cell's DNA, the information that determines what all living things look like.