I have, for the past few years, been using the following definition of a customer:
Anyone whose actions affect your results
I have found it to work in just about any situation. (It helps explain why vendors and employees are customers, too. Not to mention bankers and municipal authorities.)
Before blogging became all the rage, Tom was posting book reviews and Observations (essentially early blog posts) to this site. You can find the archives below.
What we're talking about
on the front page.