The body of the offence; the essence of the crime.
It was a general rule not to convict unless the corpus delicti can be established, that is, until the dead body has been found. Instances have occurred of a person being convicted of having killed another, who, after the supposed criminal has been put to death for the supposed offence, has made his appearance - alive. The wisdom of the rule is apparent; but in order to insure justice, in extreme cases, it may be competent to prove the basis of the corpus delicti by presumptive, but conclusive, evidence.
My criminal law professor had a more succinct definition:
I like his better.
...the law comes alive...
For those of you who, like Steve, were confused by my post, the backstory on this post is as follows:
My professor came into class and said, "For those of you who might be confused, I'm going to show you what corpus delicti is."
He grabbed a plastic shopping bag from behind the podium and started opening it, saying, "If you don't have a strong stomach, you might want to look away."
He produced a doll with a knife through its neck, exclaiming, "This is corpus delicti," and showed it around to the class.
We all cracked up. Probably the funniest incident all semester.