In the workplace, "disability" literally means "can't work." One "goes on disability."

Yet people with disabilities - according to the new paradigm of Modern Disability - are undeniably more able to work than ever before in history. More and more, "disability" does not equate with inability. More and more, having a disability need not equate with failure to perform - any more than for anyone else who is properly hired into a position they are qualified for.
So there are different things going on here. There is indeed such a thing as a disability that precludes someone from doing what they might have been doing before. A diamond cutter who loses her vision certainly is disabled in the context of that job. She is not going to be able to identify an accommodation or an adaptive strategy to continue performing that "essential task" (in human resources parlance) of the job.
But her blindness does not preclude her ability to work. In other words, disability is contextual. Given the right job in the right setting with the right set of tasks, given the tools she needs, her blindness becomes a non-issue. Just ask Kareem Dale, White House Disability Policy Advisor, and an accomplished attorney - who happens to be blind.
Often it is an artificial obstacle that disables, not the physical, sensory, or cognitive characteristics of the person (as in paralysis, blindness, or depression, for instance). As a wheelchair user, it was the presence of steps and the absence of a usable bathroom that would have prevented me from working. My disability is not a disability at all in the information-based workplace. Get me a desk and a computer, and my options are immense. The ways in which I can contribute are substantial.
So in the paradigm of Modern Disability, the word has a whole new meaning, or better put, a range of meaning. It has to be understood in the context of how that disability interacts with the overall environment, how people are capable of functioning with or without adaptation to perform what is required. Their disabilities have to be understood in terms of who they are as people, how they have prepared for the work they choose to pursue, and what their visions and goals are for themselves.
All of this is true regardless of the disability, the significance of its impairment, whether visible or invisible, from childhood or acquired, or whether the person even identifies with having a disability.
We're pretty much stuck with the word itself, so we at least have to bring its connotation up to date, and not fall into the automatic assumption that a "disability" means "inability." In the current state of the disability story, that just ain't true.