Depends. Libertarian gave the legal definition, which I think is similar to other states as well. The popular definition, as used on the homesteading forums and in the homesteading magazines, is simply someone concerned with growing some or most of their own food, and generally trying to be somewhat self-sufficient. Homesteaders range from trailer-park dwellers growing food in containers on their smaller-than-postage-stamp space, to people with large acreages. Usually a homesteader isn't making a living off his or her efforts, but many do at least pay expenses by sale of some product from their labors. If they are making a living, they are more likely to call themselves farmers. Few farmers today would qualify as homesteaders, though (with exceptions, of course).
Kathleen