Colonization is the process by which inhabitable, usually neighbouring territories that do not belong to any country can be incorporated into an existing state. There are a number of distinct and very different ways that territories can be colonized.
From the interior of North Africa to inland Scythia and western Hibernia, there is a lot of land that is selectable and usually populated, yet uncolonized and seemingly empty on the political view. Currently owned territories might also become unowned and colonizable if they are completely depopulated, particularly through 
slavery during wars. These uncolonized territories might and often do, however, have pops with their own religions and cultures. 殖民 和 定居 是一個國家可以控制這些地塊的兩種辦法, but countries with access to the 
Construct Border Fort unit action can also use that to colonize territories.
Note that uninhabitable and impassible terrain, including most 
barbarian strongholds, cannot ever be colonized.
Colonization
A territory is colonized by moving one pop of any social class, except slaves, to the territory. This can be achieved by pressing the Colonize button in the territory view of the uncolonized territory and can be done at any time, as long as the following conditions are met for at least one owned territory:
- Is adjacent to or within 2 sea tiles of the target territory
- Has at least
8 pops - Has a dominant culture that is
integrated (including the primary culture) - Has the state religion as the dominant religion
- Has at least one pop that is not a
slave (note: this non-slave pop does not have to be an integrated culture or the state religion) - Has not been used to colonize in the last year
Colonization also requires and costs 
10 gold.
When the Colonize button is pressed, one non-slave pop (which may or may not be of an integrated culture or the state religion) moves to the colonized territory, and the territory is added to the colonizer's state.
Notably, newly colonized territories do not have any colonization cooldown and can themselves be used as a base to immediately colonize another neighbouring territory, even on the same day they were colonized. This means that, if the country has a lot of gold, large amounts of territories can be colonized in a single day by constantly moving enough slaves (or 
tribesmen if the country is tribal) of an integrated culture and state religion into the newly colonized territory to bring its population up to 8 and give it a dominant integrated culture and state religion, making it immediately eligible to colonize another adjacent territory. This is possible mainly because most uncolonized provinces usually have at least one non-slave pop (usually a tribesman) which can be used to colonize. Moving all these pops, however, can be quite expensive and is generally only viable for at least moderately-sized countries.
Settling
Migratory Tribes have the ability to turn up to 20 pops in a territory with at least 3 pops and the primary culture (but not other integrated cultures) as dominant into migration units, for a base cost of 
-8 stability (certain laws and the level of centralization modify this cost through the 
migration cost modifier), with each pop becoming one cohort. These 
Light Infantry cohorts can use the 
settle unit ability to settle in any owned or uncolonized territory, turning into tribesmen of the primary culture and religion. For uncolonzied territories, this can done as long as they outnumber the existing pops, no matter how close or far the territory is from the country's current territory (if it has any), and immediately grants ownership of the territory to the settling country. Like colonization, all pre-existing pops are kept with their culture, religion, and class.
Migration units do not require military access to cross borders, and enjoy reduced attrition. Settling also works with foreign land a tribe currently controls in a war.
Construct Border Fort
Countries with access to the 
Italic Tribe traditions (which requires a primary or integrated Italic culture) can use the Construct Border Fort unit ability to colonize adjacent provinces once they have unlocked the ability in the tradition tree. This action can be taken by an army with a 
general and at least 
5 cohorts, and for uncolonized territories requires that the territory is adjacent to a currently owned territory and does not have neighbouring owned territories that already have a 
fortress building. Constructing the border fort costs 
1500 manpower and will give ownership of the border territory as well as automatically constructing a fortress and creating a 
freeman pop of the primary culture and religion there. The army that built the fort can be used to construct new border forts as soon as it moves to a new neighbouring uncolonized territory as long as the country has enough manpower, though if adjacent territories are to be colonized, the just-built fortress will need to be scrapped first.
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