Definitions are never easy. This is why when you look at a dictionary, there are so many definitions for each word. Also when you move from one culture to another there is a different cultural idiom with a different commonsense view of the world. With that said, I think most cultures will agree that heroism is good thing.
Within this word will be certain universal qualities, such as honesty, courage, and sacrifice. To the best of my knowledge, these three qualities are nearly universal. No society would see dishonesty as a virtue. Integrity is something that all people aspire towards. Similarly, courage and sacrifice are praised. More to the point, heros are people who possess courage and sacrifice to a greater extent. They are "super human" in a sense, because they are a cut above the average person. They have the ability to put other ahead of themselves. It is this sacrificial courage that is praised. Some examples would be firemen who run into fire to help victims like in 911 or solider who stand in harms way for the liberty of others.
Heroism is the qualities and actions of a hero or heroine; bravery, nobility, valor, etc.
Heroism is accepting a risk or sacrifice to achieve something noble