How to define "Religion" as a whole?

I'm currently writing an essay for an introductory religious studies class, and one of the topics that was given to me was defining what religion is exactly. Now, this isn't as easy as writing a succinct sentence that explains what Christianity or Islam is. It has to define all the world's religious groups while not including things like superstitions that aren't part of a religious tradition, philosophies (though some could argue that Buddhism is a philosophy), etc.

Or at least, as close as ever-changing society will allow.

Some things to consider:
--Not all religious traditions have a deity from a higher realm of existence. Most of these are from so-called primal religions AFAIK.
--Not all religious traditions involve worshipping or otherwise paying homage to a deity to be rewarded.
--Not all religious traditions have eternal punishment of sorts. Sure, you could degrade in standing, but nothing like Christ's Hell.

Currently, one definition I've found that's looking very good is this: "Religion is orientation to ultimate reality." I can't stop there because for one thing -- and this is my biggest complaint -- is that taken out of context, one could mistake that as atheism, which is an opposite of religion, or irreligion. There are also several other things I could point out, but I won't say just yet for the sake of hearing your thoughts.

So Southperry, what do you think a suitable definition of religion should include and/or exclude?