c. 1600, "Low Germany and the Netherlands," from the Latin name of the territory occupied by the Belgæ, a Celtic or Celto-Germanic tribe that in Roman times occupied the area below the mouth of the Rhine, including modern Belgium and much of northeastern France. Adopted 1830 as the name of a new nation formed from the southern part of the former United Kingdom of the Netherlands.

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Definitions of Belgium from WordNet

Belgium (n.)
a monarchy in northwestern Europe; headquarters for the European Union and for the North Atlantic Treaty Organization;
Synonyms: Kingdom of Belgium / Belgique