What is a Thunderstorm's Anvil?

Below, I have posted about the United 777's encounter with severe turbulence in the anvil of a thunderstorm over Missouri earlier this week.

I have been asked, "what do you mean, 'anvil'?" Good question.

Here is a drawing of a blacksmith's anvil. Note it is flat and wide at the top.

Here is a photo of a thunderstorm (technically, a cumulonimbus cloud) with the anvil shape. We refer to the large flat cirrus top of the storm as the "anvil."

UPDATE: Here is a view of a thunderstorm taken from my back yard a few minutes ago. If you ignore the building, you can see an almost perfect anvil shape.