The definition has to be subjective. If
you think that you should initiate an evacuation and you have heard nothing from the flight deck then go ahead.
I would rather you sent the passengers down the slide when it was unnecessary than left them to burn/suffocate because you hadn't received an evacuation order.
You may be the only person with access to the critical information. If there is not time to communicate you must act.
The Captain who berates you for initiating an evacuation is really upset with himself for not communicating his wishes well enough.
