Short definition
In relay interpreting, a booth doesn't interpret from the original but from another booth's interpretation. This mediating language is called the relay language.
Detailed explanation
No interpreter speaks every language. If someone speaks Finnish, for example, and no booth has an interpreter with Finnish, the relay principle kicks in: a booth that understands Finnish (say, Finnish→English) serves as a bridge. All other booths then hear not the Finnish but this English interpretation, and interpret further from it into their own languages.
The mediating language – English in this case – is the relay language (also called the "pivot"). The advantage: even rare languages can be covered without needing a dedicated interpreter for every combination. The downside: a small additional time delay occurs, and quality and accuracy depend heavily on the relay booth – errors carry forward. That's why relay booths are staffed with particular care.
Relevance in this subject area
Relay is the standard tool for multilingual conferences. It's what makes large numbers of languages manageable at all – but it requires careful planning of the pivot languages.
Practical example
At a congress with Japanese, Arabic, and six European languages, everything runs through English as the relay language: the Japanese and Arabic booths supply English, from which the remaining booths interpret further.
Related terms
Relay booth, Original audio, Passive language, Simultaneous interpreting, Interpreting system