Hi
I ll give u some real time explaination - show stopper....
consider tat u r testing some online transaction process
Test action \expected result
1.enter user name and password \ main web page should open to type acc no of the person whom u want to send money amount etc.....
2.Then click send \ confirmation and checklist window should open to get assurance from the user
3.click ok \ The sys should receive msg tran process completed succ..
The application may receive some runtime error in the first step itself tester may not able to excute next step(2)... (tat is he cant do further testing)
this defect is called show stopper .i mean to say high severity bug...