Vespucci sailed on Spanish and Portuguese expeditions to the New World in the 1490s and early 1500s, and was among the first Europeans to argue in print that the lands Columbus had reached were not Asia but a separate continent. In 1507, cartographer Martin Waldseemüller labeled the new landmass 'America' in his honor on a printed world map.
Vespucci was not a ship's captain but a Florentine merchant's agent who had helped outfit Columbus's later voyages before joining expeditions himself along the coast of South America. His published letters, some possibly embellished by editors eager for sensational reading, described a 'Mundus Novus'—a New World distinct from Asia, contradicting Columbus's own insistence that he had reached the Indies.
The claim mattered enormously: European scholars were still trying to reconcile new geographic reports with Ptolemy's ancient maps, and Vespucci's account offered a clean, marketable answer. When Martin Waldseemüller and Matthias Ringmann produced their enormous 1507 world map in Saint-Dié, they credited Vespucci by inscribing 'America'—a feminized Latin form of Amerigo—across the southern continent.
Vespucci likely never knew the honor would stick; he died in 1512, a decade before the name spread definitively to the northern continent too. Later historians have debated fiercely whether Vespucci deserved the tribute more than Columbus, but geography's accidents of naming, once printed on enough maps, tend to become permanent regardless of merit.