In Europa Universalis II, a colonist is a kind of state agent that is used to create or expand colonies and trading posts. The number of colonists your country currently has is indicated in the status bar at the top of the main map, by a number located next to a blue ship icon.
Normal colonist generation happens at the start of each month: a country will either gain no colonist that month, or it will gain one. The minimum rate of gain is zero; a negative rate of colonist gain will be displayed, but you do not lose colonists. If you hover over the colonist icon at the top of the screen, you'll get a tooltip showing the yearly number of colonists you get, and the breakdown according to the following factors:
|+2 to -2||Innovative domestic policy||+2 - 0.4*innovative|
|+2 to -1||Naval/Land domestic policy||if (land<5), +2 - 0.4*land; else 0.2*(5 - land)|
|+1 to -1||Mercantilism domestic policy||+1 - 0.2*mercantilism|
|+1||Shipyard||+1 if you own at least one city with a shipyard.|
|+1||Christian state religion||+1 for Protestant and Orthodox|
|+2||Christian state religion||+2 for Reformed, Catholic, and Counterreform Catholic|
A country is a "frontier country" if it has a land connection to a colony, trading post, or vacant province. Countries which do not own a port, and which are not frontier countries do not receive colonists. This is often the case with Central European powers such as Austria, Bohemia, and Hungary.
It is also possible to gain colonists via events. A few countries have scripted events which give them colonists. The random events Rush of Colonists, Colonial Dynamism, Conquistador, and Explorer all give some colonists, and Regional Population Boom gives some if the country selects the "a" choice. Colonists can be lost by event, but there are no random events in vanilla EU2 which cause colonist loss.