It's a good question.
I can't offer any definition that will show exactly where the line lies. But 411A has given a good example.
Many engine failures are decidedly unnerving, and can threaten the safety of the aircraft. That in itself does not make them catastrophic.
I would suggest that a failure that threatens immediate and total loss of the aircraft would be catastrophic, but not much else less than that. Even most cabin fires are not immediately catastrophic, although they can develop that way if not handled correctly.
As 411A says, there is nearly always time to communicate. As was outlined by several people on the thread in RP, in almost every case the first action by CC should be to check with the FD. If you get no response (I don't mean no coherent answer - they may be busy - I mean no response - silence) and, in your judgement, the safety of the passengers is in immediate peril (by which I mean that even a few seconds delay will cause loss of life), then by all means get them off. Don't spend the rest of your life waiting for the captain to tell you to do it.
If, on landing, one of the maingear legs collapses, the resultant noise, vibration and unusual attitude of the fuselage will be very unnerving. All else being equal (i.e. assuming no fire or other failure), a rapid disembarkation would probably be called for. But you would not know the cause of everything you see, hear and feel. It is, however, a relatively minor accident. Despite all the shock value, you should still be communicating with the FD, maintaining CRM, following SOPs, maintaining the crew team structure and teamwork.
I think I would summarise "catastrophic" as being more than damage to the aircraft, as being total loss of the airframe; uncontained and uncontrolled fire; not much else.