This site seems to be one of the more authoritative sources for descriptions of disabilities in the U.S.
Even though this isn't a government site, most of the content on this site was originally on the website of the National Dissemination Center for Children with Disabilities (NICHCY), a project funded by the U.S. Department of Education. However, as of 2014, NICHCY lost its funding. Much of the content on the NICHCY website was migrated to this site.
Most government agencies still refer to the CPIR site.
This site was recommended by students in SPE304. It's maintained by the U.S. National Library of Medicine. Depending on the disability you're researching, you may want to look at Learning Disabilities or Developmental Disabilities (in the Related Health Topics section).