1. The human mind, that is, that thinking thing lodged behind your eyes;
2. The essence of humanity;
3. The essence of that which makes a person good and decent;
4. The quality of sentience or human intelligence;
5. All of someone's personality or what makes them unique;
6. Some mystical version of a person that lives on after the body dies;
7. A spiritual concept, created by God (or the gods if you prefer) or a part of him;
8. The quality of being alive;
9. Anything else you can think of along these lines.
Note that it is also up for debate whether or not non-human animals, or for that matter, plants, have souls; feel free to adjust these answers accordingly.
How strange a thing this is! The Priest telleth me that the Soul is worth all the gold in the world, and the merchants say that it is not worth a clipped piece of silver. ~Oscar Wilde
A sensible man will remember that the eyes may be confused in two ways - by a change from light to darkness or from darkness to light; and he will recognize that the same thing happens to the soul. ~Plato
The force of mind/body/spirit that has always existed and will always exist.
Torch: Yep, its music for my soul. Makes me high.
Slash: Omg, not brown sugar pouring out. Its a song you douchebag.
Torch: Hey you don't see me insulting your religion.