In mathematics, a coefficient is a multiplicative factor that belongs to a certain object such as a variable, a basis vector, a basis function and so on. Usually, the objects and the coefficients are indexed in the same way, leading to expressions such as
a1x1 + a2x2 + a3x3 + …

An important coefficient in mathematics is the binomial coefficient.
Another meaning of coefficient is that of an important number that characterizes some property of a technical or scientific object.