Mental health crisis:

An acute reaction to an event (or series of events) whereby there is:

Significant physical or emotional distress
Psychological impairment or disturbance in people’s usual functioning.

The characteristics of a mental health crisis include:

A disruption to a person’s state of psychological balance and
The failure of their usual coping mechanisms.

A psychological critical incident:

An event is an event or series of events with the potential to trigger a mental health crisis.

Sources for (Psychological) critical incident definitions: ISO 22330; International Critical Incident Stress Foundation

Crisis mental health care:

Structured psychological crisis intervention and short-term psycho-social support by peers and/or MHP trained in trauma-responsive psychological first aid. This type of support is the psychological equivalent to physical first aid and provided to people experiencing a mental health crisis.

Factors that can influence the level of impact an event has:

When we experience an event, there are many different reasons why it might impact us, and why one person finds they have been more affected than another.

Factors that can increase the psychological impact of an event include:

Perceived risk: The degree to which a person felt they were at risk of harm
Self-efficacy (Mastery): Past experience of successfully dealing with a similar situation
Mental health: how psychologically vulnerable we were at the time (e.g. pre-existing stress, low mood, anxiety)
Relationship to person impacted: The closer we are the more we may be affected
Trauma history: Having unresolved and similar difficult experiences from the past
Relatability: How easily we can personally identify with the situation and/or people involved.

As these factors vary from person to person, and because certain factors, such as psychological wellbeing, fluctuate over time; each person can be impacted differently, and the same person differently as different times of life.