What was the “shot heard round the world”?

On April 19, 1775, the American Revolution began in Lexington, Massachusetts. The "Shot Heard Round the World" took place in Concord later that day. From the article:

"What was the “shot heard round the world”?
On the night of April 18, 1775, hundreds of British troops set off from Boston toward Concord, Massachusetts, in order to seize weapons and ammunition stockpiled there by American colonists. Early the next morning, the British reached Lexington, where approximately 70 minutemen had gathered on the village green. Someone suddenly fired a shot—it’s uncertain which side—and a melee ensued. When the brief clash ended, eight Americans lay dead and at least an equal amount were injured, while one redcoat was wounded. The British continued on to nearby Concord, where that same day they encountered armed resistance from a group of patriots at the town’s North Bridge. Gunfire was exchanged, leaving two colonists and three redcoats dead. Afterward, the British retreated back to Boston, skirmishing with colonial militiamen along the way and suffering a number of casualties; the Revolutionary War had begun. The incident at the North Bridge later was memorialized by Ralph Waldo Emerson in his 1837 poem “Concord Hymn,” whose opening stanza is: “By the rude bridge that arched the flood/Their flag to April’s breeze unfurled/Here once the embattled farmers stood/And fired the shot heard round the world.”

Emerson penned “Concord Hymn” for the dedication of a battle monument at the site of the North Bridge. At the dedication ceremony on July 4, 1837, a group of townspeople sang the poem’s 16 lines to the tune of a traditional hymn called “Old Hundredth.” Emerson, a Boston native born in 1803, spent portions of his childhood in Concord (where his grandfather, a minister, had witnessed the 1775 battle at the North Bridge from his nearby home) and moved there permanently in 1834. He went on to become one of the country’s leading intellectuals and lived in Concord until his death in 1882.

In addition to the American Revolution, the “shot heard round the world” became associated with other historical events, such as the 1914 assassination of Austria’s Archduke Franz Ferdinand, which helped trigger World War I, and the 1951 game-winning, three-run homer by the New York Giants’ Bobby Thomson against the Brooklyn Dodgers; thanks to Thomson’s “shot,” the Giants nabbed the National League pennant."