The difference is internal. The meltdown is being flooded with too much stimulation (lights/sounds/current brain capacity). The panic attack is irrational thoughts overwhelming a person. They could look similar from the outside. The difference for me is the panic attacks cause uncontrollable fear. The meltdown can be fixed with just being in a quiet environment for awhile. (About 10-15 minutes seems to work for me but I'll be unable to reenter the environment as it was) the panic attack will keep happening until I'm able to figure out what the irrational thought(s) are (self discovery usually never happens here I usually need these pointed out to me most successfully through a therapist)

While I agree with the other comment, I just want to point out that sometimes one can lead into the other. For example, sometimes I'll have a panic attack and if the environment I'm in is too stimulating and I can't get away, it'll turn into a meltdown. Vice versa; sometimes I'll have a meltdown and while I'm in such a state I don't have as much control over my anxious thoughts so they'll appear and it'll turn into a panic attack. These combination ones, in my experience, are the hardest to recover from. It takes me a full day to recover from them. Whereas, it only takes me leaving the situation and then about an hour of doing something else to recover from a meltdown and it takes a nap for me to calm down from a panic attack.