Tertium comparationis1

A non-linguistic, intermediate form of the meaning of a ST and TT. The idea
is that an invariant meaning exists, independent of both texts, which can be
used to gauge or assist transfer of meaning between ST and TT

This can be described graphically as follows:
ST chunk TT chunk
comparator
This has long been a thorny issue in Translation Studies and no one measure
has ever been accepted by all..