Disability, difference, and impairment: What's the difference?

If you've spent much time in the autism community, you've probably seen people arguing about whether autism is a difference, a disability caused by autism itself (an "impairment"), a disability caused by society, or a disability caused by both. So, what do these terms mean, and why does it matter? 

To make these issues clearer, let's look at some examples that have nothing to do with autism.

At one time, left-handedness was considered bad. Children were punished for writing using their left hands, and forced to write with their right. This occurred until quite recently, and still seems to occur occasionally in the U.K. President Obama reports that, as a child in Indonesia, his teachers hit him with a ruler for writing with his left hand. President Obama explains that left-handedness was viewed as impolite in Indonesia, and a variety of other negative associations exist. In cultures that developed a practice of eating, interacting and handling food with the right hand while cleaning their bodies with the left, the left hand was considered "unclean" and regarded with disgust. In Christianity, Jesus sits at God's right hand, while those who have lost God's favor are described as "sent to the left."

A disability is when people's characteristics do not fit with the society in which they live, which leads to bad treatment, diminished opportunities, and ultimately, less success in life. See, for example, the Americans with Disabilities Act's definition of "disability," whose last sentence hints at the important role of society:

An individual with a disability is defined by the ADA as a person who has a physical or mental impairment that substantially limits one or more major life activities, a person who has a history or record of such an impairment, or a person who is perceived by others as having such an impairment. 

Left-handedness was then a disability, for several reasons. First, left-handed people were viewed and treated negatively. Second, the environment was not accessible to them: everyday tools and furniture (e.g., desks) were set up for right-handers, making them difficult for left-handed people to use. Lastly, the educational system was dedicated to eliminating left-handedness. 
Today, many of us see this discrimination against left-handed people as backward. We realize there is nothing inherently wrong or harmful about using the left hand. If society had not chose to view left-handedness as bad, it would not be a disability. Left-handedness is not an impairment, something that would make it hard for people to do important activities no matter how they are treated. It is simply a harmless difference. Choosing to view a trait like left-handedness as an impairment makes it into a disability. 
However, if we could talk to the teachers who punished children for using their left hand, we would probably find that they believed left-handedness was an impairment. They probably thought they were doing the child a favor. If they could just make the child more like the right-handed majority, he would never face stigma or lose access to jobs and relationships. In other words, they probably were like many teachers and clinicians working with disabled children today. They saw that left-handedness was a disability, and instead of blaming the true cause--society--and trying to change it, they blamed the trait and set out to eliminate it. The only difference between those teachers then and special educators today is whether their culture considered left-handedness an impairment or a difference. 
That should make us alert to the possibility that some of the disabilities we currently consider impairments are just differences, like left-handedness.  Our children will probably look back at our treatment of stuttering, learning disabilities, autism, or ADHD with the same disgust and pity we have when considering how our ancestors treated left-handed people.
That, in fact, is the great insight of the neurodiversity movement--that people come with a wide variety of traits, and that this diversity makes the human species healthier and more likely to survive. While it's natural for humans to devalue minorities, like left-handed people, the world would be worse off without them in it. Because we are prone to view all differences as impairments whether or not they really are, we should be cautious about attempts to eliminate their disabilities--especially if it involves treating them in ways we would never treat a non-disabled person. Furthermore, the more subjective the impairment (e.g., "inappropriate" social interaction), the more skeptical we should be.
Some things we currently consider impairments may really be impairments. Even so, stigmatizing people and limiting their access to the world only makes their impairments worse. A person who cannot walk can still move around at will and live independently if there are wheelchairs, sidewalks and curb cuts, but could not before the Americans with Disabilities Act (ADA) made these technologies widespread. 
It would be hard to imagine any impairment that could cause as much suffering as being excluded from school, fired from jobs (if you are even hired at all), unable to access transportation, denied the right to vote, forcibly sterilized, forbidden to enter into contracts, or denied medical treatment. This was the normal state of affairs for people with disabilities before the ADA, and some of this treatment continues today.
One reason many people with disabilities like the term "disability" is because it acknowledges the importance of society in making people disabled, while embracing the whole spectrum from those who are a little different to those who are severely impaired. This invites people with a variety of disabilities--and attitudes about their own disability--to work together. (I have seen fewer parents of disabled children who are comfortable with the term "disability"). 
In short: A difference is a harmless trait, like left-handedness. An impairment is a trait that makes important activities difficult or impossible. A disability can be either a difference or an impairment. The distinction matters. However, for both, the way society treats people with disabilities may be what holds them back most.