In Civilization VI, a unique unit is a special unit that is only available to a specific civilization (once they discover the appropriate technology or civic, of course). Some leaders also have unique abilities that give them access to unique units; in specific cases one player can have access to two unique units in his/her game (for example, if you play America with leader Teddy Roosevelt in his Rough Rider persona)! All unique units' identities have historical grounds in their civilization (and/or their leader, if that's the case) - their names and models, and in some instances their special abilities, are based on these grounds.
Unique units have special advantages that make them superior to standard units. These could comprise higher base stats (higher CS or RS, more movement points, etc.), and/or special abilities that are truly unique (such as the Canadian Mountie's ability to build National Parks). Prior to Gathering Storm they also didn't require strategic resources to build. In Gathering Storm most unique units require resources to build and maintain, but they always require smaller amounts of resources than their common counterparts.
Note that, unlike in Civilization V, unique units do not always replace standard units. When they don't, they often fill a niche for each player that no other civilization/leader can fill at the time, giving that player a distinct advantage over his or her rivals. However, these standalone unique units cannot be pre-built and upgraded into, so they require more Production investment. Note also that unique units may belong to different eras than those they replace, which is important when considering era-specific policy cards or the effects of Great General and Great Admiral.
There are rare instances in which a unique unit of one civilization can be controlled by another one:
- Genghis Khan has a chance to capture unique cavalry units when he defeats them with his own cavalry units.
- The Sea Dog can capture unique naval units when it defeats one.
- A civilization that levies the army of a city-state can upgrade its levied units into their unique units. These unique units retain their unique identity when the city-state regains control of them, and can be levied by another civilization that becomes the Suzerain of that city-state.
- In Heroes & Legends, Hunahpu & Xbalanque can resurrect defeated unique units to fight under new leadership.
- In Barbarian Clans, Barbarian camps can train unique units of other civilizations who are not present in the game, which can then be hired by any major civilization at a Gold cost.
Despite all of these instances, there is currently no special ability or method that can be employed for a civilization to build another civilization's unique unit from scratch.