Inclusion in education refers to making sure students with physical, behavioral, or learning disabilities are included in general education classrooms as much as possible. It also means providing them with the support and accommodations they need to succeed alongside their peers.
The Individuals with Disabilities Education Act does not use the word inclusion. However, it does use the term LRE or least restrictive environment.
LRE means that, to the maximum extent appropriate, school districts must educate students with disabilities in the regular classroom with appropriate aids and supports, referred to as "supplementary aids and services," along with their nondisabled peers in the school they would attend if not disabled unless a student's individualized education program (IEP) requires some other arrangement. (Wrightslaw)