What's your personal paradigm for hills?

I used to think the road ahead became a hill whenever I had to shift to an easier gear, but then hills go down, too, and I don't shift to an easier gear to go down. Plus, if I can go down fast enough, the "hill" ahead is not hard at all and I don't shift. So it's not about shifting.

I ride in urban areas, so there aren't too many things that look like hills. There are some clear cut whoppers of hills, like Texas Street and the road out by Torrey Pines, but the rest is pretty flat -- with the occasional block or two that dips up or down. Are those hills?

What constitutes a hill? How do you define it?