1. A being with extraordinary physical or mental powers, far beyond the range of normal human ability, who uses these powers to protect the innocent and for the general good.
2. One who is granted such powers by some external means, such as advanced technology or magic, and uses them with similar intent.
Superman is a superhero because the yellow sun gives him incredible powers. Iron Man wears an armored suit with advanced weapons and abilities. Batman is the goddamn Batman.