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Supplementary Type Certificate (STC)
From SKYbrary Wiki
An approval to carry out an aircraft type modification which is deemed by the Airworthiness Authority concerned to be a sufficient change to the original design of an aircraft type to require a specific validation. For example, an Operator might wish to install an in-flight entertainment (IFE) system on their aircraft, which would involve complicated modifications to the electricity supply system on an aircraft. Alternatively called a Supplemental Type Certificate (STC).