The technical definition of maintenance refers to the process of performing functional inspections, maintaining, repairing, or replacing relevant devices, equipment, machinery, building infrastructure, and supporting utilities in industrial, commercial, and residential installations. Over the course of time, term has evolved to encompass several wordings that indicate a variety of behaviours that are both cost-effective and necessary in order to maintain equipment operating; these actions take place either before or after a breakdown.
The phrase "maintenance, repair, and overhaul" (MRO) is often used to refer to the duties of maintenance, and the term "MRO" may also be used to refer to "maintenance, repair, and operations." The vocabulary of maintenance and MRO has started to converge into a more uniform form throughout the course of time. The Department of Defense of the United States of America use the following definitions:
Any action—such as testing, measuring, replacing, adjusting, and repairing—that is designed to maintain or restore a functional unit in or to a defined condition in which the unit can execute its needed functions in order to keep the unit operational.
Any and all actions taken to either maintain a material in a serviceable condition or to bring it back to a state where it can be used effectively. Inspecting, testing, servicing, determining whether or not something is serviceable, repairing, rebuilding, and reclaiming are all included.
All of the actions that are performed to maintain a force's supply and repair capabilities so that it can continue to carry out its mission.
Maintenance is the work that must be done on a regular basis in order to keep a facility (plant, building, structure, ground facility, utility system, or other real property) in such a condition that it may be used continuously, at its original or designed capacity and efficiently for the purpose for which it was designed.
Maintaining a product or technological system is inextricably linked to the use stage, which is where the notion of maintainability must be introduced. Maintainability is defined as the ability of an item to be kept in or restored to a state in which it can perform its required functions while subjected to specific conditions of use and making use of specific procedures and resources. This definition applies to the scenario in which we are discussing maintainability.
In some fields, such as aircraft maintenance, the terms maintenance, repair, and overhaul also include inspection, rebuilding, alteration, and the supply of spare parts, accessories, raw materials, adhesives, sealants, coatings, and consumables for aircraft maintenance during the utilisation stage. Other fields, such as computer maintenance, do not use these terms. In the context of international civil aviation, maintenance refers to the following:
The action of carrying out the duties necessary to maintain an aircraft's continued airworthiness, which may include any one or combination of the following: overhaul, inspection, replacement, defect correction, and the implementation of a modification or a repair.
This term encompasses all operations that, according to the laws governing aviation, call for the issue of a maintenance release document (aircraft certificate of return to service – CRS).