An irregular noun is a noun that does not follow the forms generally accepted for nouns within that language. The most common form the irregularity takes in English is the plural form, but in other languages, the irregularity can include morphological changes such as gender, peculiar noun stem forms and irregular declensions. Each irregular noun may follow it's own set pattern of changes or may have morphological changes unique to that one word.
The most common reason for a noun to take on an irregular form is because it has been incorporated into a language from another. In such cases, instead of adapting the noun to the forms of the language, the word has been allowed to keep its original forms. A good example of this is the noun ‘datum.’ Datum is a singular word meaning ‘information’ from Latin. The plural form of ‘datum’ is not ‘datums' in the regular English fashion, but ‘data,’ which is the regular Latin plural form.
Related to this idea is the merger of dialects to form a national language. Again taking English as an example, the ‘s’ plural has become the plural of choice for nouns, but in times past, other plurals have been used such as ‘en’ and ‘eth,’ leaving irregular nouns like ‘child,’ which has the plural form ‘children.’ These occur when, for whatever reason, the language does not change the plural form of a word from one to another.
Another reason for irregularity is that the regular change to the noun’s form, whether in terms of gender or number, changes the word in a form that does not sound correct or makes the word hard to say. This might explain why there is one sheep and two sheep, rather than two sheeps.
Another irregularity in English nouns is the possessive. The regular possessive adds an apostrophe and an ‘s’ to the noun. For example, this creates “the conductor’s baton.” When the word ends in an ‘s’ such as ‘census’ and when plural nouns are at play, there is an irregularity with how the word is treated. Some people add just an apostrophe to the final ‘s,’ while others follow the regular form and add both the apostrophe and the ‘s.’ This means Charles possessing something could make it Charles’ or Charles’s depending on the writer.
An irregular noun in Latin is one where the irregularity of nouns is not limited to number and, for some reason or other, does not fit into the existing five categories of declension. These original five categories are divided by the word ending of the original stem. There are a number of reasons this might occur. Some words, for example, will decline only in the singular form or only in the plural form. These are irregular because some forms of the word simply do not exist.
Latin also has six nouns that do not decline at all. The first three are ‘fas,’ meaning fate, ‘instar,’ meaning likeness, and ‘mane,’ meaning morning. ‘Nefas,’ meaning abomination, ‘nihil,’ meaning nothing, and ‘secus,’ meaning coitus, are the other three. Each is an irregular noun because it is only available in the nominative and the accusative singular form.
In Latin, it is also possible for a word to become an irregular noun because it is heterogeneous. This means the noun has no fixed grammatical gender and that the ending of the noun stem will change. This, therefore, changes the declension of the noun depending on the gender attributed to it. Some nouns also change gender when moved from the singular form to the plural form, while others will change meaning if put into plural.
Irregular nouns in Hungarian change because some nouns have stems that alter the suffixes attributed to them when inflected. Other stems will change depending on the suffix being applied to it. For example, the Hungarian word for strawberry is ‘eper.’ If the suffix being attached begins with a consonant, then the stem is unchanged, but if the suffix begins with a vowel, then the final vowel of the stem deletes itself. The plural form of ‘eper’ is, therefore, ‘eprek’ rather than ‘eperek.’