I read in a brochure put out by a state or national park (which I believe contained both a small lake and a large pond which was bigger than the lake; somewhere in northern New England) that if a body of water was shallow enough to have ground-rooted plants all the way across it, it was a pond; if it was deep enough it did not, it was a lake. It seems there should be an area to consider as well, as otherwise a well would be a 'lake.' I was interested to read it because it was around the time 'On Golden Pond' was out, and that body of water is obviously good sized.
Also, I grew up in Oklahoma, and after the Dust Bowl days the state government said in effect 'never again' and subsidized farmers building bodies of water to the point that basically they were paid to make lakes, so that virtually every farm has a minimum of one body of water, and many have more; in school they taught us that Oklahoma then (late 50s?) had more miles of freshwater shoreline than Minnesota. It glitters wonderfully if you fly over in the early morning.