What is a contract?

Thanks to our colleagues over at CrimProf Blog, a reference to an article about law and The Simpsons.  A sample, involving the show where Homer sells his soul for a donut and “law taking guy” Lionel Hutz defends him:

Devil Flanders: I simply ask for what is mine.

Hutz: That was a right-pretty speech, sir.  But I ask you, what is a contract? Webster’s defines it as “an agreement under the law which is unbreakable.” What is unbreakable?  Excuse me, I must use the restroom.