A tantrum: child may be making eye contact or at least checking with you to see your reaction. A child having a tantrum about wanting a biscuit is likely to stop if the biscuit is offered, or if they need to continue with the rage, will switch to another demand instead (think toddler WANT BISCUIT x10, offer biscuit and watch the switch to WANT JUICE NO JUICE WANT JUICE)
Meltdown: child is so totally overwhelmed by the situation that you can’t reach them at all. It doesn’t stop when they get the biscuit or the juice or whatever it is. It may not be about anything simple but a response to external stimulus - motorbike driving past of helicopter or flickering fluorescent light or the smell of perfume. It’s not happening to get a reaction from you; you might just as well not be present at all. Can be extreme - head banging, hands over ears and screaming at the same time, eyes may be closed, child can be totally unreachable.
Tantrums are about boundaries, about working out limits, about trying to be in control sometimes. They’re about Big Feelings and usually as a child gets older they learn self control. Older children screaming for ice cream or adults stropping over the length of time they have to wait in a queue or why they need to keep 2m distance, that’s still tantrum. Husband stropping because dinner isn’t ready when he comes home. Loss of self control.
Meltdown is frequently an autism or sensory processing issues thing. Not exclusively, of course. it’s an uncontrollable reaction to the overwhelmingness of whatever sensory overload is going on.
I’d personally prefer it if people didn’t describe a perfectly normal toddler tantrum as a meltdown as I see them as two entirely separate things. But I wouldn’t jump down anyone’s throat about it. But i will get incredibly cross with anyone who decides my child’s meltdown is just naughtiness or tantrum. And if people don’t understand the difference between them it’s easy to see how that happens.
Toddler tantrum, mostly I can ride out, commenting a lots about oh dear, I can see how hard this is for you, or tickle them out of it, or pretend to fall over and slapstick comedy them out of it. None of that will reach a child (or adult) in a meltdown. Just have to protect my child’s head and my legs and ride it out; for us squashing him helps as they make him feel as though he’s flying apart so a great big bear hug helps him to join back together. For others though touch would be incendiary. It’s very individual.