Catholic ecumenical councils include 21 councils over a period of some 1900 years, which met for the purpose of defining doctrine, reaffirming truths of the Faith, and extirpating heresy. While definitions changed throughout history, in today's Catholic understanding ecumenical councils are assemblies of patriarchs, cardinals, residing bishops, abbots, male heads of religious orders and other juridical persons, nominated by the pope. Participation is limited to these persons, who cannot delegate their voting rights. Council decisions, to be valid, are approved by the popes.