Terra incognita (abbreviated TI) is latin for "unknown land"; in EU2 it is used to describe all provinces (include sea zones) which a country has not yet discovered. A particular province not discovered by a country is, to it, an unknown province. A related concept is an unknown country: when the capital province of a country is terra incognita to a second country, the first country is unknown to the second.
A country's maps are the inverse of its terra incognita: maps are the set of known provinces.
This article lists the rules around terra incognita and maps; if you looking for a discussion of these rules there's an article on how to get maps that might help you. If you are looking full maps of the EU2 world with province IDs, there is an article on maps.
Permanent Terra Incognita
There are two kinds of terra incognita. Provinces which are unknown are one type; this type may be called "unknown provinces" or referred to without qualifiers as "terra incognita". The second type is so-called "permanent terra incognita" (or PTI), which are areas in the world which in actual history were not explored during the EU2 timeframe, and are therefore "out of play". These areas do not contain provinces, and cannot be moved into, owned, or used in any way during play.
Effects of Terra Incognita
When a province is unknown, it shows on the map as all white. A player mostly cannot see what is happening there, even if he would otherwise be able to. A player cannot select the province. Note that it is still possible to see the edge of an unknown province where it borders a known province, and this makes it possible to use some of the overlay map modes to determine if the province is owned, and usually who owns it.
Movement times on both land and sea into unknown provinces are much longer than normal. There is one exception: movement by naval units into an unknown port requires the normal amount of time. Movement is not allowed into unoccupied terra incognita with certain exceptions.
A country can make no diplomatic action with an unknown country.
If a country does not know of enough foreign countries, it suffers technologically from an isolation penalty.
A rebel-controlled province cannot defect to a country if the country does not know about the province.
Discovery of Terra Incognita
There are several ways to discover terra incognita.
Exploration by Movement
A country will discover any province whenever it moves a military unit into it, via land or sea. When a country discovers a land province by movement, and the province is owned by an unknown country, it will also discover the province containing the capital of the unknown country. If a unit is disallowed from entering the destination province (because it is owned by another country), then the move will still discover the province; however the unit will remain in the original province.
All military units may plot moves into occupied provinces, regardless of tech levels or leadership. Unoccupied provinces may be moved into only by explorers and conquistadors, until naval tech 27 (fleets) or land tech 31 (armies).
Before naval tech 18, explorers occasionally discover coastal provinces when they enter an adjacent sea zone. The chance of this happening seems to vary, with the more ferocious natives making a province harder to discover. Note that leaving a port counts as entering the sea zone; it is thus a good tactic to loop explorers in and out of a port to very quickly discover any adjacent provinces that you're interested in.
At naval tech 18, all fleets automatically discover all adjacent coastal provinces any time they enter a sea zone.
Each time a country wins a sea battle, there is a small chance that it will "steal the rutters" (maps) of the losing country. Doing this will reveal to the winning country a small number of provinces which the losing country knows.
The diplomatic action "exchange maps" will cause every occupied province, and all sea provinces known by either country to be known to both.
Sacking a Capital
Whenever a country captures the capital of a non-pagan country, it gets knowledge of all provinces known by the victim. This includes capturing the capitals of friends or neutrals, not just enemies, but only the country which controls the siege gets the maps.
Joining a War
When a country joins a war against an unknown country, it gains knowledge of the capital of the country (thus making it known).
Owning a Center of Trade
Discovering Sea Zones via Port Provinces
When a country successfully creates a new colony in a port province, it discovers the sea zone which the port opens into.
When a country builds one or more new ships in a port, it discovers the sea zone it opens into.
Ownership Change of a Province
Whenever a province's owner changes, each country that knows about the province will gain knowledge of the capital province of the new owner. Changes in ownership happen via settlement (from non-owned to owned), peace treaties, defection, etc.