A human being, or other form of life, with mental and/or physical abilities beyond that of the average human capability.
It began in World War I, in the depths of Germany, when the war was drawing to a close. Oh, to be sure, the concept of such powerful beings had existed long before the war, preserved through old stories of the gods and of beings so great and powerful that they nearly destroyed the world in and of themselves. But it would be the war that changed things, the reports of an entire village liberated by a woman who strode across the battlefield alone and smote all who stood against her. Wonder Woman, they called her, and so it was that she made people hope. She made people fear. And it was from that fear that humanity began to develop the gas that would form the super soldier serum, turning ordinary men into beings far more powerful than any man had dared to dream.
When the war ended, the super soldiers disappeared, but that did not bring an end to the fears. Oh no. The gates had opened, and now it was that mankind raced to obtain a new level of power. Incidents began to appear. Strange incidents. Incidents where it seemed people had powers beyond that of a mere human. Mutant, they whispered, alien. And always, always at the edges lurked strange people who were with the government, dressed in uniform and prepared with an explanation.
In time, the world began to calm. These "powered folk", these metahumans - and oh, how they spat the word, as though it was a slur, a summation of everything these people were, blanket condemnation for those who were different than they were - slipped back into the shadows. They were a rarity, something to hate when they appeared or marvel over, as though they were little more than Bigfoot or the Loch Ness Monster. And if some people remembered the threats that had come before, the fears wrought by these metahumans and the destruction brought by the battles of these so-called X-Men, well, the world was content enough to disregard them until the day came that they couldn't.
Aliens attacked New York in 2012 and no longer could the Strategic Homeland Intervention Enforcement, and Logistics Division keep things quiet. The Chitauri had been too destructive, too memorable - too public. But then, the time of heroes had been brewing ever since they first began to reappear again, the caped crusaders of darkened streets and local heroes of fairly isolated, contained little cities. SHIELD had known it was coming, the day that they could no longer hide. Every new savior and every new doom had brought that point home. Now there was nothing to do but to live in the open and support the new team, the Avengers. Earth's Mightiest Heroes, or so they would have you believe.
What they did not know was that the Chitauri and the Tesseract - the energy source that had in part fueled the madness that had come - had in turn sent out a beacon through the cosmos, painting a target on Earth. First came the all-powerful being known as Apocalypse, awoken from his slumber deep within the tombs of Egypt after his defeat in ancient times. The X-Men prevented his rise, but to do so, they were forced to sacrifice the anonymity that SHIELD had worked so hard to preserve in the decades since the War. A man from another planet, General Zod, laid waste to Metropolis and Gotham both in a public battle with Superman that drew public spectacle at a time when the world needed it least.
Threat after threat after threat rained down upon the world; it seemed like it was accelerating as well, with the period of peace resetting for less and less time after each defeat of evil. And then came the worst blow: SHIELD itself had fallen, having been corrupted from the inside out by the organization known as Hydra, the very one who had experimented upon mutants and spurred the first super-powered arms race all those years ago as the Nazi science division. If it were not for the quick action of the Avengers, and that of the heroes around the world, countless thousands would have died as a result of the corruption. And so it was amid public outcry that SHIELD was forced to officially close its doors. They could no longer serve as a protection for the heroes they had once guarded.
The world would cleanly feel the void left behind by SHIELD, however. It was left reeling in the battle against the creature known as Doomsday, brought to life by the machinations of Lex Luthor and stopped by the two guardians of Gotham and Metropolis, Batman and Superman. It was the biggest attack known to date, and it made the public afraid. Worse, it prompted yet more metahumans from the woodwork, heroes and villains alike. It was within that fear of more - of the kinds of individuals that they could not stop or control - that the incident at Midway was covered up, the Suicide Squad formed to give the government leverage where they'd lost it.
NOTE: JUSTICE LEAGUE SPOILERS AHEAD. IF YOU WOULD LIKE TO AVOID THEM, PLEASE SKIP THIS PARAGRAPH ENTIRELY. But the status quo would not survive for long. Steppenwolf and his Parademons invaded the Earth again, seeking out the mysterious Mother Boxes, and he very nearly succeeded in his long-ago task of destroying the world. He was stopped by the Justice League, a brand new group that formed in the wake left behind by SHIELD, but there was a cost. Separating the Mother Boxes would not come without consequence.
The worlds shifted that day. A great deal of energy was released forth, and the backlash from the universe came swiftly. It snapped through the world, across the worlds, and in that moment, the energy reached some souls and spread through them. It created weaknesses in the fabric of the universe, and when the world snapped back into place again, it dragged with it the souls near them, beings from other universes, and stranded them within this world.
The Age of Heroes would not be put off any longer, and this time, it would balance the forces of good against the rise of great new, terrible evils. With the scales set and each day revealing yet more metahumans coming out of the woodwork, just what will become of this ideal world?