# Why is Dirichlet's L-function called "L-function"?

In analytic number theory, the function

$$L(s,\chi_m) = \sum_{n=1}^\infty \frac{\chi_m(n)}{n^s}.$$

is called the Dirichlet L-function and has many important uses in the study of prime numbers. In particular, if $\chi_1$ is the trivial Dirichlet character, we have the identity

$$L(s,\chi_1)=\zeta(s).$$

Why were such functions called "L-functions? What is the functional significance of the letter L?