Classical Era growth-enhancing building.
The progress towards creating the next citizen in each city is measured in Food points, adding into a Food Basket. The Aqueduct transfers 40% of the Food points required to create the last citizen, carrying them over directly into the next Food Basket. So, for example, if you needed 100 Food to produce your last citizen, you'll jump-start the production of the next one with 40 Food instead of starting at 0 Food. This effectively decreases the amount of food a city needs to increase in size by 40 percent. Build Aqueducts in cities that you want to grow large over time, or in cities where Food is scarce.
Note that the Tradition Social Policy branch provides free Aqueducts in your first four cities upon completion, so don't build Aqueducts in the first four cities if you're pursuing this policy!
An aqueduct is a system for moving water from a water supply at one location to another location, historically to irrigate crops away from natural water bodies. The word derives from the Latin for "water" and "to lead," and the aqueduct is most traditionally associated with the Romans. The most famous Roman aqueduct is the Pont du Gard in France. But the Romans were not the only civilization to build aqueducts! Magnificent and complicated aqueducts were built all over the world, including at Ninevah, capital of the Assyrians, and the Aztec capital of Tenochtitlan in North America.
The principles of aqueduct planning and construction are the same today as they were millennia ago, and humans continue to move water great distances to make our lives easier.