I would say culture is all that which is not nature. This is wonderful! And accurate, I think.

Having taken several Sociology courses, my thinking turns to that def. first. There are so many different levels! (She said, going in the opposite direction from Branny's succinct generalization.) There is "British culture" and "American culture", for example. But wait! Is there really? Do all Britons have the same culture? No! (And, did I get you to think that I was going to leave that sweeping statement as a perfect target? wink )
Everywhere, I think, there are sweeping likenesses (gender, religion, skin color, rural, urban, etc.) And also less-wide ones (education level, single parent households, homes with a person who requires constant care). But we can narrow cultures down, too: people who play clarinet, people who read King Lear smile.
Every one in these (and other) cultural groups has at least something in common with the others in that group.
But can we take culture, like language, as specific to an individual? Not an idiolect but an idioculture?