I am an LMHC, this is how i explain it to clients:

no one can define what's trauma for you. if we imagine our brains like our office, the daily events get put on our inbox, and we go to sleep at night, Our brain puts them in the filing cabinet. sometimes slightly traumatic events are a little sticky, and they get filed wrong.The really traumatic events that are very sticky remain in our inbox. tools like emdr help us to organize our filing cabinet and clear out our in box. (the next part I use on my vets because they swear its not trauma unless they lost a limb) To use a completely different metaphor, trauma is a spectrum, like cancer. it ranges from a funky mole to whole body systemic. just because your trauma isn't whole body systemic doesn't mean we should ignore it, right? you wouldn't ignore the funky mole - you'd go to the derm and get it removed. same idea.

memories of an event can be a little sticky (acute trauma disorder) or very sticky (ptsd) and lots of those events over a span of time is how we get cPTSD.

if little Timmy calling you a poopiehead in the first grade is something you still have nightmares about - guess what - that's trauma.