According to the Concise Oxford Dictionary, an amalgamation is a derivative of the word amalgamate, meaning

  1. To combine or unite into one form, or
  2. In Chemistry, this denotes a metallic alloy with the element mercury, often used in tooth fillings.

Thus an amalgamation is a combination of things, or a mercuric alloy.

The use of mercury amalgams in tooth fillings is subject to some controversy, due to suspicions poisonous mercury could "leak."

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