Civilized nations are nations that have westernised. All nations that aren't uncivilized nations are civilized. They have prestige and research advantages over Uncivilized nations and may become great powers or secondary powers, as well as having the ability to build factories.
The term may also refer specifically to a westernized nation that is not a great or secondary power
Compared to uncivilized nations, civilized nations (without higher status) have some advantages:
Initial civilized nations
These are the nations civilized at the start of the game.
All nations in North America except Hawaii are civilized.
All nations in South America are civilized.
All nations in Europe (except the releaseable Crimea) are civilized.
No independent nations with a capital city in Asia are civilized at the start of the game. The Philippines are, if released, westernized.