CSA is a Customer Service Agent.

I put my "CS"A in quotes because to many of them the concept of "Customer Service" is a title only. Walk up to one some day and see if they even acknowledge your presence. And if they do it will be with the look and tone that your are interrupting something else much more important that they should be doing instead of serving a customer. They are especially rude to crew members in uniform and even more so if you are 'express'.