The definition of a hero is

a. universally understood.

b. determined by personal and cultural factors.

c. clearly defined by a set of established traits.

d. static and impartial.

The definition of a Hero is actually more determined by personal and cultural factors B.

It would be hard to say that what a hero is is something that is understood universally given the fact that we have such a diversity in cultures. People often don't consider that what might be considered a heroic act in one culture, doesn't necessarily count as a heroic act in another culture.