Nucleus

Nucleus fundamentally means "kernel" or "central core." In science, it has many uses. The most familiar are:

In biology, the region of a eukaryotic cell which usually looks dark and round under the microscope. It was identified and given the name long before it was discovered that it contains the chromosomes, which in turn contain the cell's DNA.

In physics, the central part of an atom which contains neutrons and electrons.

Nucleus is pronounced NEW-klee-us. The plural is nuclei, pronounced NEW-klee-eye.

Some other uses of the word in science include:

In astronomy, the bright, central part of a comet;

In meteorology, the tiny grains of dust which induce water vapor to condense out into raindrops or snowflakes;

In medicine, some parts of the brain are called nuclei; for example, the nucleus caudatus.