The difference between accessibility and usability is as
Usability testing is a type of software testing which tests
a software application on level of usability or easiness that an end user will
have in using the application under test. During usability testing, the end
user is monitored by the tester or researcher, for ease of their task
completion, any issue or problems being faced.
Accessibility testing is a part of usability testing and
involves testing of application under test, for being usable by people with disabilities
as in age, motion, hearing, vision, even literacy. Accessibility testing is done manually or by
using automated tools.