What does Pagan mean to you?

It depends on whether you're talking about historical usage or modern usage, but I think both of those have been pretty well defined by the other posters. I tend to use the modern definition, since I refer to myself as a Pagan. I follow a polytheistic religion, with a focus on personal responsibility, honor, and respect for nature, that started in pre-Christian Europe. To be more accurate, I follow a reconstruction of an older religion, since many of the details of that religion were lost to history. When among other Pagans, I refer to myself as Celtic Reconstructionist, but among non-Pagans I often say either just "Pagan" or "Celtic Reconstructionist Pagan" just to make it clear that my religion fits under that umbrella.

in it's modern usage, Pagan is an umbrella term, covering quite a diverse range of religions and spiritualities, which feature either honour of nature, or the spirits of the ancestors, or both. some have Gods, others don't.