This is the question asked by Tilman Steinert & Martin Jandl in a letter to the journal Psychopharmacology.
They point out that in the past 20 years, the word "antipsychotic" has exploded in popularity. Less than 100 academic papers were published with that word in the title in 1990, but now it's over 600 per year.
The older term for the same drugs was "neuroleptics". This terminology, however, has slowly but surely fallen into disuse over the same time period.
To illustrate this they have a nice graph of PubMed hits. Neuroskeptic readers will be familiar with these as I have often posted my own and I recently wrote a bash script to harvest this data automatically. Now you too can be a historian of medicine from the comfort of your own home...
Why does it matter what we call them? A name is just a name, right? No, that's the problem. Actually, neuroleptic is just a name, because it doesn't mean anything. The term derives from the Greek "neuron", meaning... neuron, and "lambanō" meaning "to take hold of". However, no-one knows that unless they look it up on Wikipedia because it's just a name.
Antipsychotic, on the other hand, means something: it means they treat psychosis. But whether or not this is an accurate description of what "antipsychotics" actually do, is controversial. For one thing, these drugs are also used to treat many non-psychotic illnesses, like depression, and PTSD.
More fundamentally, it's not universally accepted that they have a direct anti-psychotic effect. All antipsychotics are powerful sedatives. There's a school of thought that says that this is in fact all they are, and rather than treating psychosis, they just sedate people until they stop being obviously psychotic.
Personally, I don't believe that, but that's not really the point: the point is that it's controversial, and calling them antipsychotics makes it hard to think about that controversy in a sensible way. To say that antipsychotics aren't actually antipsychotic is a contradiction in terms. To say they are antipsychotic is a tautology. Names shouldn't dictate the terms of a debate in that way. A name should just be a name.
The same point applies to more than just antipsychotics - I mean neuroleptics - of course. Perhaps the worst example is "antidepressants". Prozac, for example, is called an antidepressant. Implying that it treats depression.
But according to clinical trials, Prozac and other SSRIs are a lot more effective, relative to placebo, in obsessive-compulsive disorders (OCD) than they are in depression (though this is not necessarily true of all "antidepressants", yet more evidence that the word is unhelpful.)
So, as I asked in a previous post: "Are SSRIs actually antiobsessives that happen to be helpful in some cases of depression?" Personally, I think the only name for them which doesn't make any questionable assumptions, is simply 'SSRIs'.
Tilman Steinert and Martin Jandl (2010). Are antipsychotics antipsychotics? Psychopharmacology DOI: 10.1007/s00213-010-1927-3