Shutdowns, Meltdowns or something else?

I can tell you the definitions that I use, but as tfw7 said, they're not very precisely defined terms, and they do seem to shade into each other, and are very closely related. So bear in mind that I'm speaking mostly about my personal experiences of them.

• Shut down. Literally that, one or more, or even all, of your brain's conscious functions just stop working or work noticeably less well than usual. A total shut-down would be where you go catatonic, possibly having some awareness of your surroundings, maybe not, but unable to act. A partial shut-down is less severe, only certain functions shut-down and others keep running or work less well than usual; for example becoming non-verbal, no sense of where you are, losing the ability to do tasks which you normally find easy. A lot of what you describe seems to fit into this category, in my opinion.
• Melt down. Like a shut-down but with a very primal "fight or flight" response. You might well get some of the functional shut-downs still, but you'll act, without meaning any disrespect, rather like a trapped animal. It may involve shouting, pushing, hitting, barging people, or running away and hiding. It may involve extreme sensory-seeking such as hitting yourself or banging your head. You may have some awareness of what you're doing, but hardly any sense of control over it, if at all.
• Anxiety Attack. Usually shows itself with heavier breathing, fast or jumpy heart, and the rush of adrenaline. In an anxiety attack, you will usually feel extremely alert, even too alert, like you're looking out for danger, and it can often make sensory sensitivities seem much more powerful. This is your body preparing for "fight or flight" because it is so anxious about the situation, and it is common for non-autistic people too. It may shade into shut-downs or melt-downs, because extreme anxiety is one of the things that can trigger them.
• Overload. I take this as meaning pretty much "any of the above", where you've reached a point that you're senses or brain have too much going on and just can't handle it any more. Some people use it to mean a less stressful event where you're struggling to make sense of things, but you're still in control. Or anywhere in between: take your pick!

After a shut-down or melt-down you will usually feel extremely drained and tired, and probably need to sleep, use sensory comforters, or stim a lot. Recovery from only an anxiety attack is usually quicker and easier, but not always.

Just to confuse you even further (I'm sorry), there another one you might hear that I might as well introduce to complete the set.

• Burn out. These are very variable, but usually like a much milder shut-down, and less sudden. This is your brain telling you that you have worked it too hard and it needs a rest - autistic people's brains usually have to work much harder than most other people's. It could be a single, hectic day that brings it on, or it could take weeks to slowly build. You'll likely feel exhausted,  and want to sleep more than usual, and you might find everyday things much harder to do, or feel extremely unmotivated. If you don't get the rest, it may carry on building and it will make shut-downs or melt-downs more likely.

You'll note with all of those that stress and anxiety seem to come up a lot. In one way or another, stress causes all of them, if you include extreme emotions, having trouble communicating with the world, and sensory sensitivities as sources of stress, which they surely are. Anything you can do which lowers the amount of stress in your life will help.

ooh yeah, burn out, I get that. "Running on empty" or "just not feeling it" were the phrases I used pre-diagnosis when anyone asked how i felt and it was that kind of  thing where it's a gradual build up.

ooh yeah, burn out, I get that. "Running on empty" or "just not feeling it" were the phrases I used pre-diagnosis when anyone asked how i felt and it was that kind of  thing where it's a gradual build up.

It's one of those aspects of autism that I wish the professionals had been able to tell me about at my assessment. It seems to be so common, and has a huge impact on autistic people's lives, yet we're expected to work it out for ourselves, and there's no recognition of what our needs are when we're burned out.

When I'm badly burned out, I simply can't look after myself properly; all of my usual coping mechanisms go out of the window. The house becomes a tip, my personal hygiene becomes terrible, I barely eat, and most of all, I simply cannot stand any kind of communication with people. I go out into the world already on the edge of a melt-down before I've even got out of the door; I can become extremely agoraphobic when I'm burned out.

Another problem is that autistic burnout can easily be mixed up with depression, and I think this is partly why professionals rarely recognise it; the observable behaviours are very similar. One shades into the other, so that if burn-out isn't dealt with, it can gradually morph into severe depression. The trouble is that treatments for depression aren't always appropriate for burn-out. For example, CBT can be a very effective intervention for depression, even for autistic people; but when you're burned out, the emotional strain of counselling could just overload an already struggling brain further, making the burn-out worse. Likewise, medication may help with anxiety relief, but is not a substitute for the rest that's needed for recovery.

Once burned out, rest really is the only thing that works; I need lots of sleep, isolation from sensory triggers, and as few chores and tasks on my plate as possible for a while. It's definitely one of those things where prevention is better than cure. If you're zapped after a day at work/school, an hour's down time on getting home each day, is much better than letting it build up, so that your whole weekend is ruined. A few short holidays throughout the year may be better than one big fortnight; so that work stress is let out over the course of the year instead of building to the point where you can't cope (this has cost me every job that I've ever had.)

Like I said earlier; identify all causes of stress in your life, and try any and all techniques for minimising stress to find what works best for you; managing stress and anxiety is the key to improving all of these "brain overload" aspects of autism. Rest and "special-interest" time are justifiable therapies for autistic people, not just "symptoms" or indulgences.