something that deviates from the common forms or rules; thus, we say an irregular fortification, an irregular building, an irregular figure, &c.

in Grammar, such inflections of words as vary from the general rules; thus we say, irregular nouns, irregular verbs, &c.

The distinction of irregular nouns, according to Mr Ruddiman, is into three kinds, viz., variable, defective, and abundant; and that of irregular verbs into anomalous, defective, and abundant.