In the blog post Definitions of "Cognitive Disability" by John Rochford, we can see that it's not so easy to define the term "cognitive disability." Given how often this term appears in accessibility statements and requirements for web sites, the author was motivated to find a clear definition. His goal:
Find a recent, functional definition of "cognitive disability" written by an appropriate U.S. federal government agency, and adopted by government agencies and education institutions throughout the country.
This goal, of course, was not as simple to meet as his statement implied:
It appears no authoritative source has published a widely-used and accepted functional definition, nor a clinical one.
His post details his search process, with hyperlinked references, along with what he actually found. His closest match was a clinical definition from the U.S. Department of Health and Human Services, Administration for Children & Families.
This is problematic when the W3C WCAG 1.0 uses the term so freely, such as in Guideline 14: Ensure that documents are clear and simple:
Consistent page layout, recognizable graphics, and easy to understand language benefit all users. In particular, they help people with cognitive disabilities or who have difficulty reading.
Who makes the final decision on whether a document is clear and simple to a user with cognitive disabilities? How will a web developer satisfy this when required to meet these guidelines by law or contractual obligation? How will a web developer protect him/herself if sued as a result?
The Requirements for WCAG 2.0 has a section titled Clearly identify who benefits from accessible content and lists users with cognitive disabilities, but provides no further information.
Without definitions or pointers to definitions of what the term "cognitive disability" means, web developers are faced with more than a moving target. This is an unknown target when it comes to working to ensure that a web site meets WCAG (version 1 or 2) guidelines.
You can find a very open-ended definition of cognitive disabilities at the WebAIM site. While this may help you as a web developer who needs to support disabled users, this doesn't necessarily mean you will comply with a definition from W3C or the federal government, perhaps because there is none.