I think this is a great question. Unlike most similar descriptors, philosopher isn't a career. There are philosophers who write philosophy or teach philosophy for a living, but it isn't the fact that they write or teach that makes them philosophers.
It is also unlike artist, poet or musician in as much as a philosopher doesn't necessarily "create" philosophy in the same way as an artist creates art or a poet creates poems.
In addition, many people within the field have held widely different opinions on what constitutes philosophy.
I would personally define a philosopher as a person who advances the work of bridging between the largest, most universal questions and the smallest, most specific answers.