What is a definition of irregular verb in Spanish?

A verb that does not conjugate according to normal rules.

For example the verb Ser (to be), which is a very common word that is used all the time, is very irregular. In the tense it is

Soy, Eres, Es, Somos, Sois and Son

However, a normal verb has these endings in the present tense;

Corr##color(red)(o)##, Corr##color(red)(es)##, Corr##color(red)(e)##, Corr##color(red)(emos)##, Corr##color(red)(eis)##, Corr##color(red)(en)##