It is a term that is extremely overused today, imho.
I define a hero as someone who puts themselves in harms way to save others with full knowledge of the danger.
An example of misuse was the female soldier who was captured by Iraq's when her vehicle got separated from her group.
First she wasn't a combat soldier and wasn't supposed to be in contact with the enemy. She didn't willingly put herself in the position to be captured she was just caught in the wrong place at the wrong time,
The first responders on 9/11 qualify, though I doubt any of them thought the towers could collapse the way they did they still knew that it was extremely dangerous and they went in knowing that the towers could come down on them.
We have had so many incredible ones like the great Audie Murphy and countless other soldiers who we owe a debt we can never pay, our freedom and country came to us through their hands, through their sacrifice. Sadly many of them died in the act and passed with only a brief spark of recognition, just as many we never heard of at all.