I hope this isn't the best description...
Depth, and sunlight reaching the bottom are factors. Okay, but how deep, what about water clarity?
After further research, I find a map of my town, I see two bodies of water nearby.
One is 172 acres, 10ft depth, the other is 6 acres, 11ft depth.
The small one is called a Lake, the big one is called a Pond.
The debate lives on!