Spanish irregular verbs
The term irregular is used to describe any word that changes its form in a way that isn't standard or like most words.
This term is most commonly applied to verbs that don't follow the general rule for regular verbs. There is a set of endings for each conjugation and each tense. The irregular verbs do not follow that pattern; that's why they are called irregular.
If a verb is irregular in one tense it doesn't mean that it has to be irregular in all tenses, for example, ser (to be) is irregular in the present but not in the future.