Sometimes, the answer is "the preparation of food". Sometimes, it's "the preparation of food with heat." While I think that this is partially true, it rules out some techniques.

I think that the a good definition for cooking is "the art of adjusting hydration levels in substances meant for consumption". Thinking of cooking in terms of hydration instead of heat goes further to explain why you need to keep foods moist during cooking (and thus, why fat in some form or another is a necessity for most preparations), and why some of the tastiest dishes don't involve heat at all (I'm a big fan of seviche).

Burning = dehydration.