In a ferromagnetic material, below the curie temperature, a domain is defined as:

In a ferromagnetic material below the Curie temperature, the material is divided into small regions called magnetic domains. Within each domain, the atomic magnetic dipoles are aligned parallel to each other due to strong exchange interactions between neighbouring atoms.

This parallel alignment of all magnetic dipoles within a domain means that each domain is magnetised to saturation, i.e., it has the maximum possible magnetisation. The domain as a whole behaves like a tiny magnet with a definite magnetisation direction.

However, different domains may have their magnetisation directions oriented differently, so the net magnetisation of the bulk material can be zero in the demagnetised state. When an external magnetic field is applied, domains aligned with the field grow at the expense of others, leading to net magnetisation of the material.

Therefore, a domain in a ferromagnetic material below the Curie temperature is defined as a macroscopic region with saturation magnetisation.