Wiktionary
source text
n. (context translation studies English) The original text from which a translation is done into another language, often abbreviated as ST.
Wikipedia
Source text
A source text is a text (sometimes oral) from which information or ideas are derived. In translation, a source text is the original text that is to be translated into another language.
Usage examples of "source text".
Each one had what Astar had described as an orbit (though the Utmost Source text referred to it as an energy harmonic) so that at any given time they were nearer or farther from one another.