A nuclear meltdown is a colloquial term for an event where the normal controlling systems are unable to perform their function, the core overheats in a now uncontrollable reaction and there is a release of considerable energy in the form of radioactivity.
A child, for whatever reason, is unable to respond to the usual methods of reducing agitation, resulting in there being a build up of emotions that becomes uncontrollable or overwhelming with a release of considerable energy in the form of crying, screaming, aggression, destruction and other commonly seen as undesirable behaviours.
Meltdown is perfectly appropriate to use in the context of both neurotypical and autistic people - the usual methods of handling the emotions haven't worked, whether it's because they are tired, hungry, in pain, overstimulated, their routine has been changed, somebody has snapped at them, they've been scared - and their emotions, like nuclear fission, have intensified until they are unable to defuse or control their response even if they wanted to. Once the energy has been released, whether it's a short event or prolonged, both NT and Autistic children are usually spent, as nuclear rods are.
Gatekeeping because you believe it reduces your child's responses to being merely that of being a spoiled brat denied ice cream for breakfast is unhelpful - if nothing else because keeping the phrase for 'yourself' puts autistic children in a separate category along with deadly technology. All children can have meltdowns. All children can have tantrums. Neither is exclusive to NT or non-NT children. Fortunately for NT children, as they get older, they tend to learn to have greater awareness and control over their reactions, as do non-NT children and adults, the latter unfortunately remain more vulnerable to them due to circumstances that might not affect others as significantly.
I'm not NT. I can be angry, frustrated and a fucking brat as much as anybody else. I can also have meltdowns, but I can see when they are likely to be triggered and try to avoid them/being with people when they're on their way. The triggers might vary, I'm certainly a hell of lot more likely to flip and have to get the fuck out if I'm trapped in a supermarket with a million people, bright lights, shit stacked in the aisles and somebody is yelling at some shop assistant because they haven't got x product in stock, but I'm a middleaged woman who has had the best part of fifty years to learn to deal with the feelings of everything being too loud, too bright, too cluttered and too much out of my control. I'm not four and completely unable to articulate why I'm feeling this way.