What is a flood plain?

Good question - and very pertinent at the moment. As far as I have been able to find out, it's one of those elastic expressions which means whatever you want it to mean. Basically it's an area prone to flooding - whatever that means. The recent extreme weather has resulted in some areas being flooded for the first time ever. Are they now to be regarded as on a flood plain? My son and daughter-in-law recently bought a house near where I live, and were rather bewildered, and a little perturbed when the searches revealed that the house was designated as being on a flood plain. Knowing the area well, I was bewildered. The only running water nearby is a small brook about 200 yards away and down a gentle but significant slope. There is no way that this could present any risk to their property, so who decided on that designation and on what grounds? Even the Environment Agency's own flood map does not suggest they are at risk. The question does of course now become of some importance, as it is possible their house insurance will be loaded as a result of this designation. Wasn't it Humpty Dumpty who said "When I use a word it means just what I choose it to mean, neither more nor less".