What is a very long standing wave on a large but limited body of water generally called?

A very long standing wave on a large but limited body of water is specifically referred to as a seiche. This phenomenon occurs when water in a confined space, such as a lake or an enclosed bay, oscillates back and forth due to factors like changes in atmospheric pressure, wind, or seismic activity. The oscillation creates a standing wave effect where there are nodes and antinodes in the water body, leading to distinct water level changes over time.

The phenomenon is distinctive from those described by other terms. A surge typically refers to a rapid increase in water level often experienced during storm surges. Wave period describes the time it takes for consecutive waves to pass a fixed point, which is a measure of regular waves but doesn't classify the standing wave behavior like a seiche. A ripple denotes small waves that typically occur on the surface of the water and are influenced by the wind or movement created by objects, which is a completely different characteristic than that of standing waves. Therefore, the term seiche is the most accurate description of the described phenomenon.