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The constant which multiplies the variable attached to it. For instance, in: 2x²+3y+z+ax, 2 is the coefficient of x², 3 is the coefficient of y, 1 is the coefficient of z (which is often not shown), and a is the coefficient of x.

Any factor of a term can be called a coefficient of the rest of the term. It is most usual to consider a number or constant a coefficient of a product of variables.