A change in how information is presented, or an alteration in how a test is administered (such as orally or in a different format) or test-taker is permitted to respond. Accommodations are made to take into account various learning and testing differences among students in order to provide equal opportunity to demonstrate knowledge or understanding.
Changes in format are understood to not significantly alter what the test measures, the academic level of the test, or the performance criteria. Reasonable accommodations are mandated by the Americans with Disabilities Act of 1990.
For further explanation:
The National Center for Learning Disabilities provides this podcast which explains the difference between accommodations and modifications.