It depends on how one decides to define the word “lake.” If we define lake as a body of water that’s either connected to or fed by a natural, surface stream and is also large enough so that it needs an outlet when it reaches full capacity, then there are numerous lakes in Europe. The largest lakes in terms of surface area would be Ladoga (980 square miles), Onega (801 square miles), Vänern (776 square miles) and Vostok (715 square miles). These four lakes add up to 3,223 square miles.
If we introduce the distinction between “lake” and “pond,” which is simply defined as an inland body of fresh or saltwater smaller than a lake, then it becomes more difficult to choose the largest body of water in Europe. One source states that Singenpoo Pond, which is located in Germany and measures 3.6 miles long and 1 mile wide, is the biggest lake in Europe because it’s actually bigger than Lake Ladoga (3.45 by 1.55 miles) and Lake Onega (3 by 1.45 miles). However, most posters on the website for this source disagree with this claim and assert their own rankings of Europe’s largest lakes and ponds instead.
Then again, there is a distinction between “lake” and “estuary,” which is where a river meets with the sea or ocean. The largest estuary in Europe would actually be the Caspian Sea (143,244 square miles), which is far larger than all of its contenders like the Black Sea (47,500 square miles) and the Baltic Sea (18,900 square miles). However, this is not a lake, and it’s also surrounded by other continents like Asia and Europe proper.