Plain English Meaning
Work done on the aircraft to fix something that has gone wrong or is no longer working properly. The repair happens because a problem has already shown up, not as part of a routine check.
Full Definition
Maintenance performed to repair, replace, or restore a component or system after a fault, failure, or unacceptable condition has been identified. It is reactive in nature, addressing a known problem to return the aircraft to an airworthy state, as distinguished from preventive (scheduled) maintenance performed before any failure occurs.
Context Anchor
Seen in discussions of equipment failures, such as a turbocharger problem, when the airplane may need repair before further flight.
Derivation
From Latin 'corrigere', meaning 'to make straight' or 'to put right'. The word emphasizes that the action is taken in to something already wrong, rather than to keep things from going wrong in the first place.