An amicus curiae (literally "friend of the court") is someone who is not a party to a case, but offers information that bears on the case, and has not been solicited by any of the parties to assist a court.
An amicus curiae brief is a learned treatise, that is, a text that is sufficiently authoritative in its field to be admissible as evidence in a court, and is a way to introduce concerns ensuring that (the possibly) broad legal effects of a court decision will not depend solely on the parties directly involved in the case. The decision on whether to admit the information lies at the discretion of the court.
AAIDD has has taken a stand in US court cases on a number of issues including medical decisions affecting newborns, school exclusion, and exclusionary zoning of group homes to the rights of defendants charged in criminal cases. A selection of AAIDD's amicus curiae briefs are presented here:
On Intellectual Disability (Mental Retardation) and the 8th Ammendment's Prohibition on Cruel and Unusual Punishment
On Diagnosis and Statutory Definition of Intellectual Disability (Mental Retardation)
On the Mandate for Integration in the Americans with Disabilities Act
On Constitutionality of P&A Access
On Due Process and the Standard of Reasonable Doubt
On Violations of Title II of the Americans with Disabilities Act