A verb that does not follow the normal rules of declension.
For example, “go” is an irregular verb. Normally, the past tense of a verb is formed by adding “-ed” (eg “fill” → “filled”). However, the past tense of “go” is not “goed” but “went”.
The conjugation of regular verbs follows a pattern, which can be learnt by heart. For irregular verbs, there is no pattern at all, and you just have to learn to conjugate each irregular verb separately.