It's a question a lot of parents ask... how do I know whether it's a tantrum or a meltdown? And there really isn't an easy answer, sometimes it can be very difficult to tell.
Understanding what a meltdown is about is the key to helping you spot them more easily. Meltdowns are not power struggles or demands for attention, they are involuntary physiological reactions to being in a situation which is overwhelming.
So kids who are having one will usually be trying to relieve the tension or escape the situation in some way - by blocking sensory input, running away, touching or hitting the place where they feel the pressure (maybe their head or stomach). They often seem scared, anxious or uncomfortable and it might be difficult for them to communicate what it is that they need or why they’re so upset.
Discipline is never an appropriate response to a meltdown, because it's not behaviour which is intentional or under conscious control. Kids experiencing this kind of physiological response need calm, reassurance and a reduction in the amount of input they're being asked to process. In short, they need an escape route.
Kids throwing a tantrum might also do a lot of these same things, but have much greater control over their behavior - they can stop crying to check if anyone’s paying attention, they can communicate ultimatums. It’s easier to understand exactly what they want and why they’re upset about not getting it because a tantrum is a performance, not a reaction.
I've summarized some of these differences into this infographic. Note that this is just a guide and not a definite list of the behaviours you'll see with either a meltdown or a tantrum. Probably one of the most important things to remember about meltdowns is that everyone experiences them differently, so there's no one single correct way to describe them.
Note that the earlier blue version of this graphic is now out of date and has been replaced with this green one, so if you're sharing please make sure that you use the latest version. Thanks!