Interpreter
An interpreter is a regex that is used to locate a particular text construct (such as a TermRef or MRGRef) in a text, and that populates named capturing groups as specified in the interpreter profile of the particular text conversion tool for which it is designed.
The specifications of the particular tool tell you which interpreters are predefined for that tool, and what the interpreter profile is that a customized interpreter may use.
Editor's note
We could perhaps add references to appropriate sections of relevant tools, so that readers can see examples of this.
The generic workings of interpreters is documented in the TEv2 Text Conversion pattern.