Definition of domain: Let us today understand the concept of domain:
Domain is a region in a ferromagnetic material where all the magnetic dipole moments are aligned.
The domains are of microscopic size of the order of 10-8 to 10-12 m3 and contains about 1015 to 1017 atoms.
Each atom of a domain has magnetic moment in the same direction and gives a resultant large magnetic moment to the domain.
I hope you have understood the meaning of domain.
Reference: This article is referred from my book “electrical engineering materials” having ISBN 978-81-272-5069-0