I was hoping to see this pop up on the recent [big list question][1] about etymology or terms and symbols. Since it has not, and I can't find an answer, I will ask:

What is the reason for the $L$ in $L$-function? I've read that the general use of the term cames from Dirichlet's $L$-functions $L(s,\chi).$ Was there any motivation behind Dirichlet's use or was it just an arbitary choice?

If so, is there any compelling reason that we keep this name other than tradition?

  [1]: http://mathoverflow.net/questions/8295/origins-of-mathematical-symbols-names/21150#21150