Any group of 10 or fewer water tiles completely surrounded by land.
Lake is considered a Water tile for moving purposes.
Lakes are terrain features found anywhere on land. A lake is every single water tile, or any several water tiles, which are completely surrounded by land, and aren't so numerous as to be considered an inland sea. The difference between a Lake and an Inland sea is that the lake has fresh water, which it provides to the surrounding tiles (besides having a different Base yield).
A lake is any body of water ten tiles or smaller in size that is completely surrounded by land tiles (any larger body of water is considered ocean). When worked, lakes provide 2 food and 1 gold to nearby cities. Lakes are sources of fresh water, allowing the construction of farms on adjacent hills and tundra. Farms next to fresh water receive an additional +1 food when the Civil Service technology is researched.