For this question, I must take time with it, as much of what I have done has been described as "heroism", despite my protestations. more on that later.

he·ro    [heer-oh] Show IPA

noun, plural he·roes; for 5 also he·ros.

1. a man of distinguished courage or ability, admired for his brave deeds and noble qualities. 2. a person who, in the opinion of others, has heroic qualities or has performed a heroic act and is regarded as a model or ideal: He was a local hero when he saved the drowning child. 3. the principal male character in a story, play, film, etc. 4. Classical Mythology . a. a being of godlike prowess and beneficence who often came to be honored as a divinity. b. (in the Homeric period) a warrior-chieftain of special strength, courage, or ability. c. (in later antiquity) an immortal being; demigod. 5. hero sandwich.

This is the official dictionary definition of a hero. Now, in these modern days, the word is applied so frivolously, that the original intent of being a hero has been manipulated and distorted into something unrecognizable. Nowadays, when one mentions a hero, one automatically thinks of Iron Man, or Wolverine, or the Incredible Hulk, etc. Now, as noble-hearted as Messrs. Stark, Logan, and Banner are, they are entirely fictional.

Another major definition of the word 'hero' is relating to war heroes. As brave and noble and worthy of honoring as our men and women in the armed services are, being a hero isn't entirely about shooting guns and being either incredibly brave or incredibly fool-hardy in an attempt to save your fellow soldiers, while attacking an army that is just like your own, except under a different flag, different ideals, and speaking in a different accent or tongue.

In my mind, being a hero isn't just for big, steroid-heavy men with not enough clothes on fighting hordes of Persians, or being brave against uncertain odds, or even being in the Marine Corps.

Quite simply, being a hero is something very simple. Being a hero means, you stand up for what you believe in, and fight for those who can no longer fight for themselves, when all common sense is telling you to be just like everybody around you, and submit to what the common groupthink view declares, when it seems as though to dare to stand will be the end of you. Yet despite what your friends say, despite what the world at large says, you stand against it anyway, and fight to protect what you believe is right, fight to protect the innocents, the wounded, the fallen. 

In that sense, every person who walks this earth, from the smallest and meekest of souls to the bravest and strongest of warriors can be defined as a hero. Each in their own small way. Each making a difference in another's life, whether it be from teaching a delinquent child how to read, and staying there and not giving up on teaching them, to diplomats waging peace between countries , to soldiers on foreign tides fighting for those they love.

Even I (surprisingly) have once been described as a hero, for what I do: spreading God's love to the hurting, the wandering, the lost and fallen, fighting to protect their souls from the forces of darkness, and making a stand against the world as I attempt to resurrect the age of Chivalry. Now, I personally disagree on this being considered heroism, as it is just how I was raised, and it is how I serve God, but I digress. May this educate you in y'all's curiosity.