Short definition
A relay booth is an interpreting booth whose interpretation serves as a bridge (relay) for other booths that don't speak the source language.
Detailed explanation
If a speaker uses a language only one booth understands, that booth takes on the role of relay booth: it interprets the rare source language into a common relay language (often English), from which all other booths then continue working. The relay booth is thus the bottleneck of the language chain – its quality determines the quality of all downstream interpretations.
That's exactly why relay booths are staffed with particular care: with experienced interpreters who render speech into the relay language reliably, clearly, and in a well-structured way. Any error or imprecision in the relay booth propagates into all other languages. A small additional time delay also results, since several interpreting steps happen in sequence. The central control unit ensures the remaining consoles can cleanly pick up the relay language.
Relevance in this subject area
At multilingual conferences with rare languages, the relay booth is essential. It enables coverage of exotic languages but demands the greatest care in staffing.
Practical example
At a congress, a guest speaks Korean. Only one booth understands it; it supplies English as the relay, from which the German, French, and Spanish booths continue interpreting.
Related terms
Relay interpreting / relay language, Interpreting booth, Central control unit (interpreting), Passive language, Simultaneous interpreting