Allistic is a neologism formed from the roots allo- and autistic, meaning roughly 'fixated on others'.
Just as we have the surprisingly useful pair neurodivergent and neurotypical, we now also have the pair autistic and allistic -- allistic being anyone that does not fall on the autism spectrum.
This term came about because while the terms 'neurodivergent' and 'neurodiverse' had been useful to refer to people on the autism spectrum as a group, it quickly became apparent that this was not sufficiently specific. There are simply too many other people who have significant neurological differences, including those with ADHD/ADD, dyslexia, bipolarity, sensory integration disorder, and many others. This was okay as far as refering to the group went -- as 'autistic' continues to lose its negative connotations as a label, it more and more fills the role 'neurodivergent' originally filled -- but did not provide an easy replacement for identifying the outgroup.
It is still far from certain that allistic will end up catching on in any circles, and it may be that another term will emerge to fill this role. However, for now, allistic seems to be best contender.
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