trauma is what happens to your body when it is overwhelmed by physical events or negative emotional events which continues to respond to them in the present. Trauma is not the event but the response your body makes from continuing to feel unsafe. When someone has trauma, it's "re-experienced in the present in the form of physical sensations and emotions that may not be consciously associated with memories of past trauma."
The memories of traumatic events can be stored in your body for any amount of time, even a lifetime, as an attempt to keep you safe in the future. When something around you triggers the memory of the pain, your body gets stressed. Your body also processes things beyond trauma, for example falling from a high place when you were a kid would equate heights with pain. This would mean your body has learned to be afraid of heights. Then later in life, when you are close to an edge, your body remembers the memory from when you were younger. This sounds your body's alarms, and your body reacts to being near an edge as if the memory is happening in the present moment. The edge is telling you that you're in danger even though you aren't.
Trauma can be anything, not just a single big childhood event. Trauma is any experience or environmental factor that triggers your body's primitive danger response system. This means that it can be big and small. Trauma is cumulative. It COULD be a single big event, or it could be experiencing neglect over time, being verbally abused within multiple events, racism, etc.
Acute trauma is a single big traumatic event, also known as big T trauma. It could be a physical attack, abuse, an accident, a medical emergency, a mass shooting, etc. It could be described to be a monumental moment that altered the course of your life.
Cumilative trauma is trauma that adds up over time, also known as little t trauma. They probably do not hit you right away, but later they affect the way you think and behave. Most people that are affected by cumulative trauma experience it over a long time, from ongoing abuse, neglect, bullying, etc.
Complex trauma is a combination of acute and chronic trauma.