The fifth complement of a given interval is its interval distance from the fifth (3/2). It's very similar to the octave complement, but makes not much sense for intervals less than a fifth (since there is no "fifth reduction"). It seems to be very useful to describe the relation of thirds.
The thought that the major third and the minor third complement or contrast each other may date well back to classical era, when triads in the form of root-3rd-P5 dominated the construction of chords, yet the term was seemingly coined by Flora Canou in September 2020. 
The following interval pairs are fifth complementary to each other