The large cloud of dust and gas that escapes from the nucleus of an active comet is called a coma.

Here's a breakdown:

* Comet Nucleus: The solid, icy core of a comet.

* Coma: The fuzzy, cloud-like atmosphere that forms around the nucleus when it is heated by the Sun.

* Tail: The long, thin stream of dust and gas that trails behind the comet, pushed away from the Sun by solar wind and radiation pressure.

So, the coma is the first stage of the comet's activity as it approaches the Sun. It's formed by the sublimation (turning directly from ice to gas) of the comet's ices, and it carries dust particles that were embedded in the ice.