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This question is adjacent to this other one.
The martian dichotomy refers to the fact that the northern and southern hemisphere are radically different on Mars, with different surface ages and different crustal thickness.
When was the phrase "martian dichotomy" used for the first time? By who?
It is always difficult to say who is "the first" at something, as there are often cases where you can really dig and find something earlier.
I started searching based on my answer to your similar question, but Hartmann (1973) did not use the word "dichotomy" in his paper. I did a search through all of the journal articles on my computer for the word "dichotomy," including all articles published in the planetary science journal Icarus since its inception, and I came across Soderblom et al. (1974) "Martian Planetwide Crater Distributions: Implications for Geologic History and Surface Processes." In it, they use the actual phrase, "The Martian global dichotomy" which is pretty close to the exact term you're looking for.
Given the response to your other question in that the earliest identification of it was from Mariner 9 images, which came in 1971–1972, I suspect that if this Soderblom et al. paper was not the first, it was among the first to use that phrase. Of course, it also takes time for any phrase or term to catch on, so when the term became common is a completely different question and one that I'm not sure could be answered other than by asking planetary scientists from that era (I know some, I could try to ask if you wanted).
