From Wikipedia: "an area of land adjacent to a stream or river that stretches from the banks of its channel to the base of the enclosing valley walls and experiences flooding during periods of high discharge"
That means the flat land around a river. It's designed by Mother Nature to flood when it rains a lot.
People who live in houses built on such land should hold the moneygrabbing developers responsible when they inevitably get flooded and invite those people along to personally bail them out. With a small bucket.
Someone said to me when I stood on the bridge over the Trent (Cloud Trail - National Cycle Route 6) and pulled out my camera, "it's impossible to capture the extent of it!"
That's right, the flooding is spectacular. I tried a couple of panoramas from the bridge, and another when I was on higher ground by Stanton by Bridge.
This is by the road just before I took off my boots and waded through flooded road
In the distance here is Swarkestone Causeway (showing the arches. Those driving across only see the tarmac top and miss the beautiful stonework). Usually the water here is grassy with animals occasionally grazing on it. It floods when there's too much water, when the water goes away the grass recovers and the animals come back. Hakuna Matata.