In Civilization and Civilization II, the aqueduct is a city improvement that enables the city it is built in to gain population above a certain level.
In both games, the aqueduct requires the advance of construction to build, and 120 resource shields. It cost 2 gold a turn in upkeep.
In Civ II, the aqueduct is followed by the Sewer System, which enables the city to gain even more population.
Civ tacticians and strategists are of different opionins of the best times and circumstances to build aqueducts in cities.