The farm ponds around Texas have always been called "tanks," but that gives the wrong visual impression to city-dwellers or folks from other areas (I think?)
Some not-very-illuminating info from EPA:
The main difference between lakes and ponds is size. Lakes are deeper and larger bodies of water that can influence local climate if large enough. Ponds are much smaller than lakes and usually have the same temperature from top to bottom, whereas lakes can have dramatically different temperatures from the surface to the bottom waters. Also, rather than affecting local climate, ponds are usually greatly affected by local conditions.
http://www.epa.gov/bioindicators/aquatic/lake-r.html Differing temperatures? I guess if you can't throw a rock across it and you want to call it a lake, it's a lake.
