The best, readily accessible, treatment of this question is the Environmental Fact Sheet Lake or Pond -- What is the Difference? put out by the New Hampshire Department of Environmental Services. "From a regulatory viewpoint there is no distinction between a lake and a pond... From a naming convention there is no precise difference between a lake and pond, although waterbodies named "lakes" are generally larger and/or deeper than waterbodies named "ponds." From an ecological or limnological perspective, there is a difference between the two. The difference, however, is somewhat arbitrary and not consistent or precise." Most other sources I've checked concur that bodies of water 8 hectares (19.768 acres) or larger and where light cannot penetrate to the bottom should be classified as a lake. However, standards and opinions do vary.
Lake or Pond??? (Maine Bureau of Land & Water Quality)