DEFINITION:

MRO may be defined as «all actions that have the objective of retaining or restoring an item in or to a state in which it can perform its required function. The actions include the combination of all technical and corresponding administrative, managerial, and supervision actions”. Maintenance, repair and overhaul (MRO) in the aeronautical industry is a complex process that has strict and precise requirements defined by airworthiness authorities to guarantee the safety of passengers and aircrew. 

(Source: http://www.ijcaonline.org/research/volume135/number12/vieira-2016-ijca-908563.pdf   and PAE own elaboration)