To me a freelancer is someone who makes all or most of their income by working contracts rather than being employed. The dictionary definition is:
A person who sells services to employers without a long-term commitment to any of them.
but to me it's more than that as well. There is an ethic behind it - I think freelancers often work harder, usually longer and unsociable hours (espcially when starting out), but essentially they enjoy what they do. It's a way of life and not just a job.