What is the solid, central core of a comet called?

Nucleus

The nucleus is the solid part of a comet, often described as a 'dirty snowball' because it is composed of rock, dust, and frozen gases. When a comet approaches the Sun, these ices sublimate to create the glowing coma and tail we see from Earth. Despite their bright appearance in the sky, most nuclei are among the darkest objects in the solar system, reflecting very little light.