Accessibility, usability, and SEO

I was reading this article today about the differences between accessibility and SEO and it made me think about usability as well. It's interesting to me how many people equate accessibility and usability - it never even occurred to me to equate SEO into that mix. But as Joe discusses it, I realize that that is something people do all the time. I define them this way:

• accessibility - making a site usable by people with disabilities.
• SEO - search engine optimization; making a site usable by search engines.
• usability - making a site usable by whomever needs to use it.