Looking for a clearer definition of 'meltdowns' in young children.

Hi everyone,

I'm hoping someone can help me to understand ASD meltdowns in toddlers. My 20 month old son is currently being assessed as possibly ASD. He has meltdowns, but they are never unexplained. What I mean by that is that he has what I would assume to be the normal reactions from a toddler when being forced to do something he does not want to do. 

If he wants to go off somewhere and is prevented, or isn't in the mood for a bath or to have his teeth brushed, or is taken downstairs when he would prefer to stay playing in his bedroom etc, he'll scream and protest for a few moments. If I'm carrying him, he'll try and push away in frustration. If he's standing up, sometimes he'll fall to the floor. Normally, leaving him on the floor and giving him the clear message that nobody is going to rise to his behavior calms him down quickly. 

Does this describe an ASD meltdown, or is there more to it? My understanding is that ASD meltdowns are much longer, spontaneous, possibly unexplained, sometimes due to some form of sensory overload. Is that correct?

Thanks.