For each person "local" can mean a different thing. For you it could mean a region, a state, a set number of miles from your home, a town, or even your own backyard. I personally think that it's a worthwhile lifestyle experiment to set a limit and test if you can live within it for a little while (a few years ago we did a month long experiment of local eating with only a few exceptions). It reveals where your food supply systems might be broken. Like maybe there are laws getting in the way of local, small dairy farmers. Maybe you live in a grain growing state but you can't buy any local grain because it makes more sense for agribusinesses to ship everything across the country to be processed, over processed and shipped back to you. It's interesting to see how the invisible web around you is working. And on a more positive note, a local eating challenge helps one truly appreciate an occasional orange that's been flown across the country.
Anyway, for the sake of the Dark Days Challenge, the winter time rule-of-thumb definition of local is a 150 mile radius. The image above is what that looks like for us here in Boulder.