Reconnaissance (often called recon for short) is the act of using units to explore the map. Reconnaissance is an important part of strategy in the Civilization games and their spin-offs.
Civilization IV[]
Scout
Recon units are those that specialize in reconnaissance. Scouts, which are available after researching Hunting, are the first available recon units, and can upgrade to Explorers after researching Compass. Both units are unable to attack, but get only positive results from activating Tribal Villages. Scouts also receive a 100% strength bonus when fighting animals.
Later in the game, Spies can be used to explore enemy territory and perform espionage missions.
Civilization V[]
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Scout
Scouts, along with Warriors, are the first units available. Their ability to ignore terrain makes them good at finding Ancient Ruins, natural wonders, city-states, and other civilizations early on. Scouts that find advanced weapons while exploring Ancient Ruins will upgrade to Archers and retain their ability to ignore terrain; however, they cannot be upgraded by normal means.
The Shoshone Pathfinder replaces the Scout.
Civilization VI[]
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Scout
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Ranger
Scouts, along with Warriors, are the first units available. Their ability to ignore terrain and the special promotions they earn make them good at finding Tribal Villages, Natural Wonders, city-states, and other civilizations early on. Or if you're lucky, finding a metor site left behind by a Meteor Shower.
Scouts can upgrade to Rangers after researching Rifling (and Skirmishers after researching Machinery in Gathering Storm), giving them increased strength and a ranged attack. In Rise and Fall, Rangers can upgrade to Spec Ops after researching Plastics, gaining the abilities to paradrop and directly attack support units in addition to extra strength and range.
The Cree Okihtcitaw replaces the Scout, the Incan Warak'aq replaces the Skirmisher, and the Scottish Highlander replaces the Ranger.