What is the biggest battle ever fought?

studied History
The Battle of Vienna
So yes, there have been large and protracted actions defined as battles that have lasted weeks, months, or years. But if we want to narrow the definition of "battle" to be defined as a single uninterrupted moment of conflict, then September 12, 1683 is a strong contender for both number of combatants simultaneously engaged, and because at 5 PM that day there occurred what could well be the most awesome spectacle in the history of battle.

The set up: For 2 months the mighty army of the Ottoman Empire had laid siege to Vienna. And though they massively outnumbered the 15,000 defenders by 10 or 20 to 1, it was no easy task. The fortifications and walls of Vienna were some of the most modern of the era, and reinforced with 370 cannon. The Ottomans were forced to slowly whittle down the defenses by completely cutting off the city and using extensive tunneling and sapper action to reduce the walls.
The Relief: Arriving on September 11, the Polish and German relief forces had managed to quickly consolidate their fractious alliance under the leadership of the Polish King. Early the next morning, as they were still consolidating and readying their order and lines of battle, the Ottoman forces attacked. At the same time, beneath the clashing infantry, a subterranean battle was being waged as thousands of Ottoman sappers in miles of tunnels faced an army of counter sappers from the city defenders.
The counter sappers managed to delay or disarm the massive bombs intended to devastate the walls. This denied the Ottoman infantry the chance to take the city, and to assume strong defensive positions within the walls.

The Charge: Late in the afternoon, after an entire day of massive infantry battle above and below the ground, the exhausted Polish-German forces let out a cheer. For they saw that their cavalry was finally ready to engage en masse. At the top of the hill abutting the battlefield, 20,000 men on horseback had formed up. Spearheaded by 3000 Polish Hussars - the most elite and heaviest cavalry of the age - they charged. Not as individual units or in bunches, but all at once. The largest cavalry charge in human history. 20,000 horses. 80,000 hooves pounding the ground in unison. A mass of steel and flesh and muscle crashing down upon an Ottoman infantry that was worn out from 12 hours of intense fighting.

The ground did tremble. A man made earthquake. And though the Ottoman forces well outnumbered the relief army in infantry forces, this thunderous, deafening wall of cavalry carved a massive hole straight through their lines. A deep gash was cut all the way through to the supply tents and camps of the Ottomans, ripping the army into fractious pieces. Faced with this massive, unprecedented attack, the Ottoman forces collapsed, and slaughter followed. And though fighting would continue into nightfall, the greatest cavalry charge in human history had finished the battle, and solidified the Habsburg dynasty's power in Europe.

Fun tidbit: Remember the epic siege of Gondor in Tolkien's "The Lord of the Rings?" Many think it was inspired by the Battle of Vienna. It has many parallels, including the massive cavalry charge that finally breaks the siege just in time. Though I think it would be fair to say the Ottoman Empire was considerably more civilized than the Orc horde, their fate was quite similar.