Per Merriam-Websters:
a crater whose diameter is many times that of the volcanic vent because of the collapse or subsidence of the central part of a volcano or because of explosions of extraordinary violence

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Note added at 7 mins (2007-01-23 20:22:23 GMT)
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Ruth may be right, but a magma chamber is this:
: the underground space usually deep below the earth's surface occupied by magma that may ascend from it to or toward the surface

And I understand the "caldera" to be the crater that is visible at the top of the volcano (visitors often visit these as a tourist attraction)