The term specially designed instruction (SDI) is embedded within the Individual with Disabilities Education Act’s (IDEA) definition of special education and is mirrored in state special education statute.
Specially designed instruction is instruction in which the content, methodology, or delivery of the instruction is individually adapted to address the student’s disability-related needs so the student can access, engage, and make progress in the general education curriculum and meet the standards and expectations that apply to all students of the same age or grade. This is true irrespective of where the instruction is provided.
For more information on "what" specially designed instruction is and "who" can provide it, see the following resources.
What is Specially Designed Instruction (SDI) and Who Can Provide It?, Wisconsin DPI
Bulletin 10.07: Describing Special Education, Related Services, Supplementary Aids and Services, and Program Modifications or Supports, Wisconsin DPI
Reading Teachers and the Provision of Specially Designed Reading Instruction as Outlined in Students' IEPs, Wisconsin DPI
College and Career Ready IEPs, Step 4: Align Services, Wisconsin DPI