What is the name of the middle matrix in a quadratic form $x'Ax$?

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In a quadratic form, $x'Ax$, does matrix $A$ have a special name? I am trying to write a sentence that looks like "Using vector $x$ and matrix $A$ as the ______ matrix, we can write the quadratic form as $x'Ax$...". I don't know if "multiplicand" or something similar is used to describe this matrix.