Visible and Invisible Disabilities
What many people don't recognize is that you can be invisibly disabled one moment, and visibly disabled the next. It all depends on context.
One way that a normally 'invisibly' disabled person can become visibly disabled is by who they are with. When I was volunteering with an ABA program for autistic kids, I saw one example of this. In one part of the program, there were both autistic and NT kids doing an integrated gym class (this was for the higher functioning kids). Anyway, one kid came out of the gym at the end while I was chatting with one of the therapists, and she greeted him. I knew immediately that he was autistic. Not because of his appearance or behavior, but because of the way she spoke to him. She used the same tone of voice as she did in ABA sessions, just with better grammar.
You can also be visibly disabled by being with other disabled people. In my disabled youth group, when we go on outings, it's evident that at least some of us are disabled. And we are clearly a group, without the kind of divides that often occur between disabled and nondisabled people, so those of us who aren't visibly disabled are assumed to be disabled as well. Note that this can mean mistaking nondisabled people for disabled people. Hearing children of Deaf parents, when out in public with their parents, are often assumed to be deaf because they're signing and with people who may be more obviously deaf. Even though they aren't actually disabled, in that setting they're visibly disabled.
Some people are visibly disabled in certain settings but not in others because those settings involve skills that they lack. Apart from with my disabled youth group, the only times I've been visibly disabled is when I get lost - especially on the bus. I don't know things most people are expected to know. I act more eccentric out of stress. In general, I seem developmentally disabled. Similarly, a dyslexic child in a class discussion may not be visibly disabled, but when reading aloud in class they are.
On the Internet, many 'visibly disabled' people become invisibly disabled. At the same time, although they may not actually be recognized as disabled, some 'invisibly disabled' people become more visibly disabled on the Internet. Higher-level language problems, receptive or expressive, are the most visible disability on the Internet.
This brings me to another point. You can be in between visible and invisible disability. There are certain stereotypes of people that are 'almost' disabled, such as geeks, stupid people, 'wackos', etc, who are generally assumed not to be able to help being like that, but aren't really considered disabled. Some people who actually are disabled are recognized just enough to fall into those categories. Also, some disabled people are mistaken for people who are drunk or high on some substance, such as people with balance problems or people who show visible signs of perceptual abnormalities. Actually, this is among the most obviously disabled you can be without being recognized as disabled.
So it's a whole lot more complex than just visible or invisible disability.