Definition
The installation of new equipment, modifications, or improvements on an aircraft after it has left the factory and entered service. A retrofit updates an existing aircraft to a newer standard or adds capability it did not originally have.
Plain English
Adding or upgrading equipment on an aircraft that is already in service, rather than building it that way from new.
Context Anchor
Seen in maintenance records, equipment installation instructions, avionics upgrades, and aircraft modification discussions.
Derivation
From 'retro-' meaning 'backward' (Latin) and 'fit', meaning to install or equip. The sense is fitting something back into an aircraft that already exists -- bringing an older airframe up to a newer fit.
Why Pilots Care
Allows older aircraft to meet current regulations, gain safety or efficiency improvements, and extend useful life without purchasing new equipment.
Analogy
It is like adding a backup camera to an older car. The car did not come with it from the factory, but the new equipment can be installed later.
Intuition Check
Retrofit does not mean a temporary repair. It means installing a later part, system, or change onto an aircraft that was already built.
Example Sentence 1
The operator chose to retrofit the older 172s with glass cockpit displays rather than buy new aircraft.
Example Sentence 2
Many older business jets have been retrofitted with winglets to reduce fuel burn.