During his third voyage to the Americas in 1498, Italian explorer Christopher Columbus discovered a new landmass in South America and named it Tierra Firme (Spanish for "mainland"), believing that it was part of the Asian mainland.
As years passed, Spanish settlers continued to explore and conquer more territories and eventually found the region rich in gold and silver. The region, at first known as Tierra Firme, was later referred to as Nueva Granada, meaning "New Granada", by the conquering Spanish.
In the sixteenth century, an Italian geographer named Amerigo Vespucci came to the conclusion that this newly discovered continent was not, in fact, a part of Asia at all, but a completely new continent. He suggested that the new continent should be called America, after his own name, but the suggestion did not find much immediate popularity.
In 1502, Italian historian and humanist scholar Peter Martyr d'Anghiera, in his account of Columbus's voyages titled "De Orbe Novo" (About the New World), used the feminine version of the name Columbus in its Latinized form, "Columbia". This would become the first reference to the newly found lands in the Americas as Columbia.
As the Spanish explorers continued their conquests, different parts of the South American continent were assigned different names. Eventually, the name Nueva Granada was applied exclusively to the region consisting of what is today Colombia and Panama. The name Columbia was also used occasionally to refer to the territory, but mostly in a poetic sense.
In the 19th century, a prominent Venezuelan politician and writer, Francisco de Miranda, began to promote the name Columbia as a reference to the entirety of the America's in his revolutionary publications, inspired by the United States of America. The name had a patriotic connotation, as it symbolized unity and independence in reference to Christopher Columbus' discovery.
In 1810, the people of the Viceroyalty of New Granada declared their independence from Spain, and the new country was named the Republic of Colombia in 1819. The name commemorates the explorer who many consider the bridge between Europe and America, and reflects the vision of a unified Latin America with a shared heritage and identity.
So, to summarize, the name Colombia is derived from Christopher Columbus' Latinized name "Columbia", which was first used by Peter Martyr d'Anghiera in 1502, and later popularized by Francisco de Miranda in the 19th century, eventually leading to its adoption as the official name of the Republic of Colombia.