I’m sure many people use the words interchangeably and they are really not even similar in terms of their nature.
Fear is an emotion that triggers a reaction.
Phobia is a reaction of accumulated and disproportionate fear.
Trauma is a disorder of the nervous system caused originally by fear.
Fear even when it’s an emotion, it can become chronic and a lens that tarnishes life. It can become an automatic response of an overactive amygdala that sees everything as dangerous. In that way is when similes a phobia. The main difference is that chronic fear applies to almost everything, while phobias are very selective and apply to a specific thing, place, action, or circumstance. Chronic fear is a symptom of trauma, while phobias are a separate phenomenon that is even considered a disorder in itself when it debilitates daily performance.
Phobias can be seen as symptoms or as a long-term consequence of PTSD. Even if the PTSD is gone, a phobia can remain as a reaction to a stimulus that reflects danger. Many phobias are irrational because the memory of what caused the fear in the first place didn’t get stored completely; only something about it kept stored in the right amygdala as a reminder of something threatening and undesirable. That’s why phobias are defined as irrational because they seem to come out of nowhere; but they always come from some previous negative experience, either traumatizing or very scary.
Trauma is a whole injury that is not as selective as a phobia. In that sense, phobias are more adaptive than trauma. Trauma (PTSD or C-PTSD) is maladaptive since it affects so many processes of the system’s functioning. It has the negative valence of bad memories in the right amygdala as well, but it also affects other parts of the brain. Phobias can also be symptoms of traumatization and of trauma since they carry avoidance as PTSD does, but PTSD has many other symptoms that people with phobias don’t present. Phobias can be part of other consequences of PTSD or C-PTSD, or can present alone without PTSD.