Abend is short for "Abnormal end."
An abnormal end is a term used to describe a software crash or if an application suddenly has a fatal error and quits.
This occurs in all kinds of software including older browsers which normally will see a crash or lockup and eventually close itself, without user input.
Abend was first used by the IBM OS/360 system as an error code for unexpected crashes. It is now used as a general computing term.