A number used to multiply a variable.
Example: 6z means 6 times z, and "z" is a variable, so 6 is a coefficient.
Variables with no number have a coefficient of 1.
Example: x is really 1x.
Sometimes a letter stands in for the number.
Example: In ax2 + bx + c, "x" is a variable, and "a" and "b" are coefficients.
See: Variable