In modern parlance, a colony is a territory subject to a form of foreign rule. Though dominated by the foreign colonizers, colonies remain separate from the administration of the original country of the colonizers, the metropolitan state (or "mother country"). This administrative colonial separation makes colonies neither incorporated territories, nor client states. Some colonies have been organized either as dependent territories that are not sufficiently self-governed, or as self-governed colonies controlled by colonial settlers.

The term colony originates from the ancient Roman colonia, a type of Roman settlement. Derived from colon-us (farmer, cultivator, planter, or settler), it carries with it the sense of 'farm' and 'landed estate'.

Furthermore the term was used to refer to the older Greek apoikia (Ancient Greek: ἀποικία, lit. 'home away from home'), which were overseas settlements by ancient Greek city-states. The city that founded such a settlement became known as its metropolis ("mother-city").

Since early-modern times, historians, administrators, and political scientists have generally used the term "colony" to refer mainly to the many different overseas territories of particularly European states between the 15th and 20th centuries CE, with colonialism and decolonization as corresponding phenomena. While colonies often developed from trading outposts or territorial claims, such areas do not need to be a product of colonization, nor become colonially organized territories.

Some historians use the term informal colony to refer to a country under the de facto control of another state, although this term is often contentious.