Monotheism is a civic in the Vikings, Traders, and Raiders! scenario in Civilization VI. For all practical purposes, it is obtained by having a majority religion in one's cities, which provides 100% of its 
Culture cost.
Civilopedia entry[]
Monotheism is, simply, the belief that there is only one god (or goddess). This belief can be dated back to Zoroastrianism, and is still found today in Christianity, Sikhism, and others.