Technically, a pond is man made, and a lake is natural, but size has long been what most use to define a lake and pond. Some areas say 5 acres plus, and other areas say 10 acres plus to be a lake. Depth can also be part of the differences in definitions. Some states define a "lake" by size (Maine defines a lake as 10 acres surface area or more)
I looked into the same question a few years ago. FWIW One definition I remember said a BOW has to be big enough to impact weather patterns to be a lake. By that definition, does that mean there are only 5 lakes in the US?
Practically speaking a lake is a BOW that is too large to afford to fix problems with, and a pond is manageable. So if you are not rich like me, then a 1 acre pond is considered a lake. ;-)