The International Civil Aviaiton Organisation (ICAO) defines an approved maintenance organization (AMO) as: "An organization approved by a Contracting State, in accordance with the requirements of Annex 8, Part II, Chapter 6 — Maintenance Organization Approval, to perform maintenance of aircraft, engine, propeller or parts thereof and operating under supervision approved by that State."
The definition can be found in ICAO Doc 9760, Airworthiness Manual, Fourth Edition, 2020.
Part 145 Maintenance Organisational Approval
Part 145 is the European standard for the approval of organisations that perform maintenance on aircraft and aircraft components that are registered in European Union Aviaton Safety Agency (EASA) Member States.