在数学中，系数是在某个表达式中作为某个对象的乘法因数的常数。比如说，9x中的系数是9。

拥有系数的对象可以各种各样，比如说变量、函数、矢量或者矩阵。有的时候系数似乎没有对象，比如说坚尼系数，实际上是因为对应的对象过于生僻而没有列出。在某些情况下，系数会被标上上标或下标，以示区分，如下式中：

为了与x_n协调，a_n 是一个带有下标的系数，n = 1, 2, 3, ……

在一个关于变量x的多项式P(x)中，x的系数一般用下标k标记。

其中下标最大的不为零的系数称为多项式的首项系数。

数学中的重要系数有二项式定理中的二项式系数。

In mathematics, a coefficient is a multiplicative factor in some term of a polynomial, a series or any expression; it is usually a number, but in any case does not involve any variable of the expression. For instance in

the first two terms respectively have the coefficients 7 and −3. The third term 1.5 is a constant. The final term does not have any explicitly written coefficient, but is considered to have coefficient 1, since multiplying by that factor would not change the term. Often coefficients are numbers as in this example, although they could be parameters of the problem or any expression in these parameters. In such a case one must clarify which symbols represent variables and which ones represent parameters. Following Descartes, the variables are often denoted by x, y, ..., and the parameters by a, b, c, ..., but it is not always the case. For example, if y is considered as a parameter in the above expression, the coefficient of x is −3y, and the constant coefficient is 1.5 + y.