As Goddess Hestia have said in the end of episode eight, Argonaut is the name of the skill Bell has achieved. She said that it is a skill of wanting to be a hero. And it proved that it is so, when it activates every time he wants to save somebody. So i think Argonaut could be define in this anime as "the hero in you" much like the literal meaning of it and in which the Argonaut skill actually gives you the power to fulfill your desire to be a real hero whenever your desire(for wanting to be a hero) wakes up.

As Goddess Hestia has said in the end of episode eight, Argonaut is the name of the skill Bell has achieved. She said that it is a skill of wanting to be a hero. And it proved that it is so, when it activates every time he wants to save somebody. So I think Argonaut could be defined in this anime as "the hero in you", much like the literal meaning of it and in which the Argonaut skill actually gives you the power to fulfill your desire to be a real hero whenever your desire  (for wanting to be a hero) wakes up.