UK definition (RCEM): It describes the sudden onset of aggressive and violent behaviour and autonomic dysfunction, typically in the setting of acute on chronic drug abuse or serious mental illness.
Australian definition (NSW Health): Behaviour that puts the patient or others at immediate risk of serious harm and may include threatening or aggressive behaviour, extreme distress, and serious self-harm which could cause major injury or death.
There are some superb resources on the Life in the Fast Lane site on this topic. Really recommend having a look!
There is a useful summary on some de-escalation strategies & techniques, from HSI here.
This handbook from a UK NHS Trust outlines some key principles from their conflict resolution training.
JRCALC Clinical Guideline: Acute Behavioural Disturbance.