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 speeds up population growth by by 40%, transferring that percentage of the stored Food points directly into the next Food Basket each time a new Citizen is born. 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. Build Aqueducts in cities that you want to grow faster, remembering that their effect will be greater in cities that have access to an abundance of Food.
Keep in mind that production occurs after growth, so if a city finishes building an Aqueduct on the same turn it grows, the city's Food store will be at 0 at the start of that turn. The best time to finish an Aqueduct is when the city has a bit less than 60% of the Food required for the next Citizen.
Note that the Tradition policy tree provides free Aqueducts in your first four cities upon completion, so don't build Aqueducts in the first four cities if you're filling out this tree!
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.