A verb that is not inflected in the usual ways. One example of an irregular verb is one that does not add -ed to the root form to create the past tense and past participle. There are several common types of irregular verbs classified according to how they indicate past tense and past participle: vowel changes (begin, began, begun); -en added (beat, beat, beaten); vowel changes (spin, spun, spun); -d changes to -t (lend, lent, lent); no change (put, put, put).
A regular verb is any verb whose conjugation follows the typical pattern, or one of the typical patterns, of the language to which it belongs. A verb whose conjugation follows a different pattern is called an irregular verb. (This is one instance of the distinction between regular and irregular inflection, which can also apply to other word classes, such as nouns and adjectives.)