What is meant by original sin?

Original sin has to do with the fallenness of human nature. Jonathan Edwards wrote a tremendous treatise on original sin. He not only devoted himself to a lengthy exposition of what the Bible teaches about man's fallen character and his propensity toward wickedness, but he made a study from a secular, rational perspective that addressed the philosophy that was widespread in his day: Everyone in the world is born innocent, in a state of moral neutrality in which they don't have any predilection toward either the good or the evil. It's society that corrupts these innocent natives, so to speak. 

As we are exposed to sinful behavior around us, our normal, natural innocence is eroded by the influence of society. But that begs the question, How did society get corrupt in the first place? Society is people. Why is it that so many people have sinned? It's almost axiomatic in our culture that nobody is perfect. And Edwards asked questions like, Why not? If everyone were born in a state of moral neutrality, you would expect statistically that approximately 50 percent of those people would grow up and never sin. But that's not what we find. Everywhere we find human beings acting against the moral precepts and standards of the New Testament. In fact, whatever the moral standards are of the culture in which they live, nobody keeps them perfectly. Even the honor that's established among thieves is violated by thieves. No matter how low the level of morality is in a given society, people break it.

So there is something indubitable about the fallenness of our human character. All people sin.

The doctrine of original sin teaches that people sin because we are sinners. It's not that we are sinners because we sin, but rather, we sin because we are sinners; that is, since the fall of man, we have inherited a corrupted condition of sinfulness. We now have a sin nature. The New Testament says we are under sin; we have a disposition toward wickedness, so that we all do, in fact, commit sins because it is our nature to commit sins. But that's not the nature that was originally given to us by God. We were originally innocent, but now the race has been plummeted into a state of corruption. Author: RC Sproul