The name given to the lake in London's Hyde Park, formed when the Tyburn River was dammed in early Victorian times.

Amusing although only tenuously related anecdote:

I used to work at a company whose offices backed on to another London park, St. James's Park. I also used to smoke, and so I'd often be standing outside the office having a quick fag. Being situated a mere stone's throw from the Houses of Parliament, Westminster Abbey and Buckingham Palace, getting asked directions by tourists was an occupational hazard. However I'll never forget the smartly-dressed Australian with whom I had the following conversation:

Aussie: Can you tell me the way to the park?
Me: (pointing at big green grassy thing on the other side of the road) Yes, it's over there.
Aussie: That's the park with the lake in it?
Me: Yes (because there's a very beautiful and famous lake in St. James's Park)
Aussie: And there's a bridge over that lake?
Me: (a little bit worried now) errr ... yes. Aussie: Good on ya mate! Now can I can go and stand on the bridge over the turpentine.

And on that note, he wandered off. To this day I haven't got a clue what he was on about...