What is the highest lake in the world?

Depends on what you call a "lake": whether a large-ish puddle would qualify, or a small pond, or something much larger...

According to www.highestlake.com, the prize would go to Lhagba Pool (6,368 m), Tibet. (http://www.highestlake.com/highest-lake-world.html) The webmaster of this site has proposed a definition of "lake", which I quote:

Quote:

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1. The lake surface area must be at least 1 hectare, or 2.471 acres. 1 hectare is 100 meters by 100 meters. Long-recognized lakes smaller than this size are grandfathered in; if ancient Hawaiians and Incas were willing to make the trek up to their sacred lake, that's good enough for me!
2. The lake must be at least 2 meters deep somewhere. You know, for swimming.
3. The lake must be mostly open water. If you can walk or hop all the way across it in your water-proofed hiking boots, it's not a lake, it's a marsh.
4. The lake must be perennial, lasting throughout the entire year. Specifically, the lake must retain at least 100 cubic meters of liquid water year-round. That's 10x10 meters by 1 meter deep - fish could live in that. It's okay for the lake to have a permanent surface layer of ice, like Lake Vostok in Antarctica, as long as there's water down below.
5. Someone has to visit the lake on the ground to verify and document it.

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However, the largest commercially navigable lake is widely recognised to be Lake Titicaca (3,821 m), on the border between Peru and Bolivia. See http://en.wikipedia.org/wiki/Lake_Titicaca

I always hate saying this, but 'it depends what you mean"! Some lakes are temporary, though regular, glacial lakes. Some are almost permanently frozen. Alternatively the world's highest navigable lake is definitely Lake Titicaca in Peru - 3,800 metres (12,464 feet). It seems that the highest body of water worthy to be called a 'lake' is Lhagba Pool in Tibet - 6,368 metres (20,892 feet).

You just beat me to it achernar - I shouldn't have answered the phone!