A minimum equipment list (MEL) is a list which provides for the operation of aircraft, subject to specified conditions, with particular equipment inoperative (which is) prepared by an operator in conformity with, or more restrictive than, the MMEL established for the aircraft type.
The master minimum equipment list (MMEL) is a list established for a particular aircraft type by the organisation responsible for the type design with the approval of the State of Design which identifies items which individually may be unserviceable at the commencement of a flight. The MMEL may be associated with special operating conditions, limitations or procedures.
Source: ICAO Annex 6: Operation of Aircraft
The MEL is approved by the operator's national airworthiness authorities.
The operator shall include in the operations manual a minimum equipment list (MEL), approved by the State of the Operator which will enable the pilot-in-command to determine whether a flight may be commenced or continued from any intermediate stop should any instrument, equipment or systems become inoperative. Where the State of the Operator is not the State of Registry, the State of the Operator shall ensure that the MEL does not affect the aeroplane’s compliance with the airworthiness requirements applicable in the State of Registry. (ICAO Annex 6 Part I Chapter 6 Para 6.1.1)
An operator may not operate an aircraft which does not comply with the approved MEL, except with the explicit permission of the Appropriate Regulatory Authority, usually the NAA. Such permission will not be granted to allow the aircraft to operate outside conditions set by the corresponding MMEL.
In most cases, multiple unserviceabilities of unrelated aircraft systems cannot be addressed by an MMEL nor, therefore, by an MEL. The decision as to whether or not to accept for flight an aircraft which has multiple unserviceabilities which would individually be allowable by MEL provisions ultimately rests with the designated Aircraft Commander, subject to guidance promulgated on a proactive or ad hoc basis by the aircraft operator.
In any case, if multiple unserviceabilities exist, the MEL should be consulted for each individual item to check if there are any incompatibilities for each of the associated dispatch conditions.
European Regulations and supplementary information concerning the MEL for fixed wing air carrier aircraft are contained in IR-OPS and EU-OPS. The FAA system is similar except that FAA considers an approved MEL to be a Supplementary Type Certificate (STC) issued to a particular aircraft by serial number and registration number as a way of providing authority to fly it in a condition other than that at which it was originally type-certificated.
The MEL is entirely separate from the Configuration Deviation List (CDL), which is a list of secondary airframe and engine components which may be recorded as missing for without prejudicing the acceptance of an aircraft for flight.
and associated Acceptable Means of Compliance and Guidance Material