What constitutes a valid marriage?

Answer 1

A marriage must usually be registered with some authority; nowadays this is usually a marriage registers set up by the governments of different countries, but in the past, it was often a religious authority (for example, the Catholic Church in many South American countries).

A marriage may be declared void (invalid) if certain conditions were not fulfilled; for example, one of the parties involved didn't know that a marriage was being carried out; or didn't freely give his agreement.

In any case, better check the specific laws for your country or state.

Answer 2

It is a marriage performed by a Catholic Priest.