Well technically psychosis is "a mental disorder characterized by symptoms, such as delusions or hallucinations, that indicate impaired contact with reality"per an online dictionary, but of course that's way off the mark. And when the dictionary is stupid, confusion is gonna happen.

Psychosis is a symptom that can include delusion(s), hallucination(s) or both.

A PTSD flashback is a type of psychosis. However, when most people ask of they have experienced psychosis, it seems like most people really want to know if they have experienced psychosis caused by schizophrenia, schizoaffective or bipolar disorder.

I've had psychosis caused by mental illness (BP 1 with psychotic features).
I've had psychosis caused by trauma issues, such as olfactory hallucinations when triggered.
I've had psychosis caused by drugs.
I've had psychosis caused by sleep deprivation and/or stress.

Psychosis is a symptom, and many people have experienced it.

For example, I once had a panic attack where I was convinced that I could not breathe and that my throat was swelling shut. This was a severe delusion, powered by anxiety and caused me extreme duress, and the people around me could not convince me of anything or talk any sense into me. Even the basic fact that I couldn't be talking if I wasn't breathing did not register with me. I had lost touch with reality and was delusional while also very panicked.

But most people would not imagine something like that ^ when thinking of psychosis. Most people imagine A Beautiful Mind or the famous schizophrenia cat painting.