A "meltdown" is a period of intense & uncontrollable emotions in which the individual loses control of their emotions and sometimes their behavior. Its almost like an epileptic seizure. Meltdowns are characterized externally by strong emotion states: rage, anxiety, extreme sadness. It can take a significant amount of time to recover from a meltdown.
This is different from being simply angry, or sad, because the condition remains until, it is thought, the brain can clear itself and 'reset'. No amount of emotional 'work' can clear the situation, it just takes time and quiet.
It is not hyper- or hypo-mania, as in bipolar disorder. It is not simply 'rage'. While bipolar mania has triggers, the length of the behavior is generally longer, and is part of the 'normal' sine-wave of the individual's emotional life. Meltdowns are short punctuations, they are interruptive, they are caused entirely by external stimuli.
Meltdowns are caused by too many inputs that the Aspie is forced to deal with. Because we have limitations of executive function, we literally cannot process all the items coming into our consciousness. When the inputs are all equally, vitally important, as in during an argument with a loved one, the stress of trying to manage all the inputs inside one's own head becomes unbearable. Often, the individual will attempt to find a way to exit the stressful situation; sometimes successfully, sometimes not. The upshot is an Aspie emotional explosion -- or sometimes implosion. This is quite startling to the observer when normally the Aspie is quiet and introverted and generally peaceful.
Trying to keep an Aspie under control during a meltdown is generally not a successful strategy. You will probably end up with some pretty wounded emotions if you insist upon interfering, as the Aspie will lash out. In children, there can be physical pushing and violent attempts to leave the scene of the conflict.
Aspie adults have meltdowns. Sometimes we understand what's happening and we try to find a safe space to deal with it. Other times, especially when its brought on by emotional conflict with another person, its literally impossible to bring it under control without at least shouting and without escape to a quiet place to calm oneself. Usually an adult Aspie can limit any physical altercation, unless another actor pushes them into it.
The main problem with meltdowns as adults is that, while we undertand perfectly well in the abstract that this is not normally acceptable behavior, even to ourselves, its often uncontrollable. We usually just want to leave.
In employment or academic settings, meltdowns can have severe consequences when teachers, school administrators or employers witness what they see as unacceptable behavior. When law enforcement gets involved, there's often a poor outcome for the Aspie that may result in prosecution, jail time, fines, as law enforcement is poorly equipped to handle an uncooperative adult suddenly throwing explitives at them.
When unscrupulous people are involved with an Aspie, they can learn how to push the Aspie into meltdown, putting the Aspie into an especially vulnerable position. This is a special favorite of manipulative people.
Controlling and inflexible people are the bane of Aspies, as they will increasingly attempt to elicit the assistance of various outside help (pastors, doctors, teachers, cops) to keep the Aspie under control.
Meltdowns often lead to psychological misdiagnosis. Often they are mistaken for evidence of hypermania/hypomania, as in Bipolar Disorder, anger issues, or much worse. "Oppositional behavior" is a favorite term of the mental health community for behavior that they can't explain without challenging the physician or counsellor's training or preconceptions.
To an outsider who doesn't understand what's going on, a meltdown looks like terribly poor behavior, evidence of poor upbringing, character flaws, mental illness. Its not. Its a human being trying to cope with his/her/their own brain shutting down to protect itself.