The word "hero" does not necessarily have a correct definition. There is no set paradigm depicting a hero's traits, and there is no instruction manual on how to be a hero. The best we can do in defining this word is slap on a stereotypical label saying that these guys are strong, smart, brave, willing to make sacrifices, etc. However, I believe that being a hero has an underlying meaning that is simply unfathomable to the point where it cannot be described with words. The best I can do in defining this word is write this essay.
The least that is expected from the archetypal hero is that they save people. However, heroic acts differ in size. Heroism could range from something as big as saving the world from a collision with a meteor or something as small as helping a friend out with homework. In both cases, the act has saved at least one person and as long as someone recognizes the act in question as heroic, I believe it counts as heroism.
Now think about how many cases of accidental heroism are out there. It happens all the time; people are saving others without realizing it. I think people have it built into them: help people in need. Albeit, there are people who would choose to ignore the fact that people need their help and therefore do nothing to improve the situation, but many other society members act on impulse: if he or she sees a person who needs help and he or she can do something about it, then he or she helps out. A hero can simply be an ordinary person who analyzes the situation, realizes they can help someone in need, and takes action according to what he or she believes will best benefit the situation.
The final point I'd like to add is that heroism, in my opinion, is mainly about perspective. What about all the people who are torn from society because they thought they were doing something for the greater good, but people simply found them psychotic, so they were locked away? Those people believe that society is now deprived of a hero, and they might find that the people who locked them up are not heroes at all, whereas regular citizens exalt whoever put the criminally insane behind bars. An additional example is the tearing down of the Astrodome. Some people thought this was great news. They probably thought it was about time for something new. On the flipside, many people thought that the Astrodome was an enormous loss, and tearing it down was an awful thing to do. People might view those who tore down the Astrodome as heroic or not depending on where they stand.
All in all, my definition is inevitably incomplete. What I will say is this: Heroes aren't born. They're not made. They are people who do what feels right to them, while society selects certain people that fit the archetypal image of "hero."