Colonization is the act, by a militarily strong country, of invading and taking over the sovereignty of another area, which then becomes known as a colony. This often includes the establishment of one or more settlements, also called "colonies", inhabited by emigrants from the colonizing power.
In ancient times, maritime nations such as Greece often established colonies. These appear to be more benign, emphasizing uninhabited land, and farming it. In classical times, land suitable for farming was often claimed by "barbarian tribes" that lived by hunting and gathering. To ancient civilized people, the land simply appeared vacant.
Another great colonization of ancient times was the Roman colonization of the empire that spread across three continents, many of current great cities of Europe were in fact Roman colonies (the German city Cologne was in fact called by the Romans Colonia Claudia)
Following the European "discovery" of the New World by Christopher Columbus in 1492, the major European powers embarked on a large-scale campaign of colonization, the consequences of which have shaped the current state of the world to a considerable extent (see European colonization of the Americas).
In science fiction, space colonization is sometimes more benign. Humans find an uninhabited planet, and inhabit it. In more recent science fiction, they may create inhabitable space (by terraforming or constructing a space habitat) and call that a "colony." On the other hand, if the planet is already inhabited, much less benign consequences ensue: indeed, some science fiction authors have used the colonization of alien planets by humans, or the colonization of Earth by aliens, to explore the real-world issues surrounding the phenomenon.
More metaphorical uses of the word include "coca-colonization", that is, the erosion of a country's indigenous culture and its replacement with corporate mass-culture, usually American in origin (see cultural imperialism).