A lake is a large body of water, usually fresh water, surrounded by land.
Large lakes are sometimes referred to as "inland seas" and small seas are sometimes referred to as lakes. For examples (of saline lakes): the Great Salt Lake, but the Dead Sea. The largest "lake" in the world is the Caspian Sea, and the deepest is Lake Baikal in Siberia. The term lake is also used to describe a feature such as Lake Eyre which is dry most of the time but become filled under seasonal conditions of heavy rainfall.
See the list of lakes.
There are dark basaltic plains on the Moon, similar to lunar maria but smaller, that are called "lacus" (singular lacus, Latin for "lake"). They were once thought by early astronomers to be literal lakes.