Alternative dispute resolution or ADR refers to a "procedure for settling a dispute by means other than litigation, such as arbitration or mediation." Black's Law Dictionary 91 (9th ed. 2009).
According to Stephen J. Ware, Alternative Dispute Resolution § 1.5, at 5-6 (2001),
"ADR can be defined as encompassing all legally permitted processes of dispute resolution other than litigation. While this definition is widely used, ADR proponents may object to it on the ground that it privileges litigation by giving the impression that litigation is the normal or standard process of dispute resolution, while alternative processes are aberrant or deviant. That impression is false. Litigation is a relatively rarely used process of dispute resolution. Alternative processes, especially negotiation, are used far more frequently. Even disputes involving lawyers are resolved by negotiation far more often than litigation. So ADR is not defined as everything-but-litigation because litigation is the norm. Litigation is not the norm. ADR is defined as everything-but-litigation because litigation, as a matter of law, is the default process of dispute resolution."
Mediation is "a method of nonbinding dispute resolution involving a neutral third party who tries to help the disputing parties reach a mutually agreeable solution." Black's Law Dictionary 1070-71 (9th ed. 2009).
Arbitration is "a method of dispute resolution involving one or more neutral third parties who are usu. agreed to by the disputing parties and whose decision is binding." Black's Law Dictionary 119 (9th ed. 2009).
Negotiation is defined as "a consensual bargaining process in which the parties attempt to reach agreement on a disputed or potentially disputed matter. Negotiation usu. involves complete autonomy for the parties involved, without the intervention of third parties." Black's Law Dictionary 1136 (9th ed. 2009).