An accommodation is a change in the standardized testing materials or procedures that enables students with a disability or with limited English proficiency to participate in an assessment in a way that measures abilities. An accommodation does not change the concept being measured. Testing accommodations are designed to “level the playing field” during the testing situation or to achieve “assessment parity” for all students regardless of disability or language deficiency.
A modification (e.g. Alternative Assessment), on the other hand, mean that the curriculum and/or instruction is changed quite a bit. When modifications are made, kids with disabilities are not expected to master the same academic content as others in the classroom.