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.

Reference: This article is referred from my book “electrical engineering materials” having ISBN 978-81-272-5069-0