Most English nouns form their plural by adding either -s (books, bands, bells) or -es (boxes, bunches, batches). These plural forms are said to follow a regular pattern. These noun forms are called regular plurals.
But not all nouns forms follow this regular pattern. In fact, some of the most common English nouns have irregular plural forms such as woman/women and child/children.
These noun forms that follow this irregular pattern in order to form their plurals are called irregular plurals.