The concept that variance in neurological structure adds needed diversity to the human race. The celebration of that diversity. Or, a word referring to the variety of ways in which the human brain can be wired. The usage varies from person to person.
The word appears to originate within the autistic community. The first widely publicized use of this term came in Jane Meyerding's article, "Thoughts on Finding Myself Differently Brained." She in turn attributes the word to Judy Singer's essay, "Why Can't You Be Normal For once In Your Life?" Judy Singer says, "I'm not sure if I coined this word, or whether it's just 'in the air,' part of the zeitgeist."
To some people, neurodiversity only encompasses human neurological variations that they define as being positive in some way as well as potentially negative. This usage generally refers to the autistic spectrum, specific learning disabilities, and similar conditions. Thus you will get lists including autism, Asperger's syndrome, PDD-NOS, dyslexia, dyspraxia, dyscalcula, NLD, attention deficit disorder, and Tourette's syndrome.
To other people, this usage is too limited. They use the term to refer not only to what is described above, but to other variations including mental retardation, cerebral palsy, epilepsy, hydrocephalus, head injury, and various neurological diseases such as multiple sclerosis.
Calling such things a matter of diversity might be unfamiliar to people outside of the disability rights movement, which largely regards disability in general as a matter of diversity. Advocates of this usage of the term are not saying that things like epilepsy don't need to be treated or that everyone should go out and get brain damage, only that the range of neurological diversity in human beings doesn't stop at conditions that people are born with or that people would normally think of as having a positive side. They are likely to have a different worldview than most people about the range of human variation. They are not trivializing anything, but rather looking at what they see as the big picture. Many will not see these things as discrete medical diagnostic entities, but will see human variation blending together from the usual to the extremely unusual in a smooth way.
Some people's usage of the term neurodiversity refers to groups of people only. Neurodiversity, to them, reflects the variation between people, including neurotypical people. Others claim that neurodiverse is an adjective that can be applied to individual people, but only those who are not neurotypical. They see neurodiversity as the opposite of neurotypicality. This may be due to a language gap between the USA and the UK, where the terms have taken on slightly different meanings.
Most people who talk about neurodiversity aim for acceptance of a wider variety of people on the planet, but what form this takes varies. Some limit this acceptance to people with more fashionable labels. They might say that people with dyslexia or Asperger's syndrome are neurodiverse and need their alternate neurology and unusual forms of intelligence respected, but that people with mental retardation are not really cool enough to be included in this. Others push for acceptance of a wider group of neurological types in society, insisting that everyone has some place in the world. For the record, I aim for the latter.
The association of neurodiversity with the more fashionable diagnoses has caused some amount of controversy over it. Some people view neurodiversity as a concept coined by people who think it's cool to have a particular label, and suspect it's a matter of "Ack! That person doesn't fit in a category! Quick, find one for them!" They wonder if the neurodiversity movement is overlooking people with real difficulties in life. There are also people in the anti-psychiatry movement who fear that the neurodiversity movement too readily embraces a neurological and medical model for all human behavior.
The answer to that depends on which person you ask. Opinions may vary. There are shallower and deeper interpretations of neurodiversity, and there can be all the snobbery and elitism associated with it that can happen in any group of people. On the other hand there are people involved in neurodiversity who are genuinely committed to acceptance of all people and the removal of barriers to our participation in society. Infinite diversity in infinite combinations.
Meyerding, Jane. "Thoughts on Finding Myself Differently Brained". Independent Living on the Autistic Spectrum. http://www.inlv.demon.nl/subm-brain.jane.eng.htm. Accessed March 17, 2005.