Invincibility is the ability to withstand any amount of physical damage, pressure, or harmful force of any kind, causing no harm or pain to the individual gifted with this power.
Invincibility doesn't make the person unable to be vanquished or able to win all battles. This was shown when Matthew took in the powers of the Sphere of Excidio. He began invincible, but the Bonum Coven vanquished him. Jeric was invincible, but not ever fight he came in, he won.
Beings with this ability tend to be stronger and have Immunity to all kinds of abilities, powers, potions (such as Adflicto), magic, spells, magical weapons (not including the Angel Sword). Their powers may be able to be stripped, however.