A "back-translation" is a translation of a translated text back into the language of the original text, made without reference to the original text.
Comparison of a back-translation with the original text is sometimes used as a check on the accuracy of the original translation, much as the accuracy of a mathematical operation is sometimes checked by reversing the operation. But the results of such reverse-translation operations, while useful as approximate checks, are not always precisely reliable.
Back-translation must in general be less accurate than back-calculation because linguistic symbols (words) are often ambiguous, whereas mathematical symbols are intentionally unequivocal.
In the context of machine translation, a back-translation is also called a "round-trip translation."
When translations are produced of material used in medical clinical trials, such as informed-consent forms, a back-translation is often required by the ethics committee or institutional review board