Irregular Words
Irregular words are words that do not follow the normal grammatical rules for conjugation. A common case in English is nouns whose plural form does not end in s or es (e.g. woman, women).
Irregular words need to be added because the search engine can't use the normal rules for stemming of the language, to make different forms of a word equal to each other.
Irregular words are created as a part of an irregular word dictionary.
Example
For English, the way of stemming words in plural form is to remove s and es endings, e.g. making trains and train equal to each other. This does however not work for woman and women, so an irregular word will have to be registered in Ecommerce Search with those two terms.