The Holding Pond, also called a Pond, is the smallest body of water in an Animal Crossing series town. There are usually at least two ponds in every town, with some towns having only one, and others having a third or fourth depending on the layout.
According to Animal Crossing: New Leaf, the water in the ponds comes from beneath the ground.
It is the only body of water that is not part of the water system, which includes the river, the waterfall, the river pool and the ocean, therefore it is limited in the number of fish that can be caught. The fish are small and not worth much. Normally, either one or two ponds will be present in front of the Museum.