There'a a huge difference between the two, recognised by parents of autistic kids which parents of NT kids won't have seen and mostly don't understand.
A tantrum is something that happens when a child can't have what they want, you can tell it's a tantrum because when you give them the desired object, the undesirable behaviour stops.
Yes, I know, sometimes their expectations aren't reasonable and you can't give them the desired thing, (No, you can't take next door's Great Dane for a walk on your own) but if you could fulfil that expectation, the tantrum would stop. In short, tantrums are seen as bad behaviour when a want is denied.
Meltdowns have nothing to do with wanting things, they are brought on by sensory overload and the child is absolutely overwhelmed with some or all of the sights sounds tastes touch, smells and intensity of lights in a particular place, to the point that they are totally disorientated and cannot distinguish one sensory input from another. The usual reaction to this is to scream and scream in the hope that the adult with them can make all the sensory overload stop.
Some children just bolt in terror, some lie down and scream, some kick and scream, but it's important to note that they aren't doing that because they desire a particular object, they are acting in that way because their senses are so overloaded they cannot shut them out and need to be removed from that location and placed somewhere more quiet and calm.
Post-meltdown, the child doesn't just return to their normal behaviour, often they are so exhausted they cannot function well and that can last into the next day or two until they recover their equilibrium.
I agree with Greysparkles, for a parent of an autistic child, it is a demeaning thing for parents of NT kids to say theirs are having a meltdown when they mean a tantrum, because they don't have a clue what an actual meltdown is as their child has never had one.
I'd also think that had this question been asked on the SN boards there would have been a lot of agreement because the posters there would immediately identify with the OP and would fully understand that this was yet another instance of their observations and daily life being minimised and trivialised by people who had not shared that experience and didn't fully understand the difference between a tantrum and a meltdown.