Since you ask specifically about Aristotle, he defined tragedy as events of a certain magnitude. We usually think of tragedy as being bigger than life, an event out of proportion with normal life, usually a terrible one.
Here is a good link:
While the definition and interpretation of "tragedy" has changed over the centuries, it always includes a protagonist who falls from grace due to a "tragic flaw"--sometimes a weakness in his character and sometimes a circumstance of which he has no control. The fact that the tragedy is expected to produce audience enjoyment creates an unusual paradox.
I think a tragedy is when the protagonist (the main character) starts off good, but ends up either a) dead or b) in a death like state. Shakespeare has some great tragedies out there!
Tragedy is simply a disastrous event either expected or unexpected
Tragedy: A dramatic composition, often in verse, dealing with a serious or a sober theme.