Trauma is defined as an emotional wound/shock that creates substantial lasting damage to the psychological development of a person. Trauma can also be described as whatever the brain perceives as threatening.
The brain and the body experience the situation the same whether the threat is perceived or real.
As you can probably imagine from the definition, a person’s experience of trauma is highly relative. What might be traumatic to one person may not be traumatic to another due to a variety of factors we will discuss below.
One of these factors is control. How much control a person had over the situation makes a big difference. If a person had no control over a situation that happened to them, then they are likely to feel more traumatized than if they were able to have at least some semblance of control during the event. This is why we often see victims of trauma expressing a deep need to be in control as it is an effort to repair this experience.
Secondly, age is also an important variable. As you can imagine, young children are more sensitive to external stimuli than are older children. Young children are also more acutely aware of their parents than are adolescents who are more sensitive to peers.
Along with age, the level of brain development at the time of the trauma is also an important consideration. 
The part of our brain that allows for rational thinking is not fully developed until a person is in their late twenties. So, if an individual experiences trauma early in their life, their lack of brain development will impact their perception and experience of the event.
The length of time the trauma occurs plays a big role as well. If the trauma occurs over the course of years versus a one-time life event, it is likely to be more impactful. This is not to say that a one-time event cannot be traumatic (it can), but perpetual trauma over long periods of time will have more of an impact than if the same thing were to happen just one time.
A person’s disposition/temperament are a big factor in determining whether or not something is traumatic. This is a variable that we have little control over as it is part of our genetic makeup. A child with an anxious temperament is likely to have MORE anxiety over an event than a child without an anxious temperament.
This probably comes as no surprise, but the amount of support/resources a person has in the event of a traumatic experience is a major factor in how it impacts them. In fact, this is one of THE most crucial components. When we go through difficult experiences alone, we carry the load entirely. When we have support, we are able to share that load. Therefore, the experience tends to be less taxing.
The level of betrayal that comes along with the experience impacts how traumatic it is. For example, abuse from a close relative or parent is likely to have more of an impact than abuse from a stranger. This is because there is a perceived level of trust that is violated when abuse comes from someone who is supposed to love and care for us.
All of these factors are why the answer to the question of what constitutes trauma is not so cut and dry.
What defines something as traumatic is a person’s lived experience of it. That includes all of the factors mentioned above. When we understand and accept the fact that many factors play a role in the way we experience a situation – as traumatic or not – then we are much more prepared to begin healing from it. We must recognize and allow space for our personal experience without judgment.