principal diagonal vs main diagonal

principal diagonal and main diagonal both are nouns.

Word	Noun	Adjective	Verb	Adverb
principal diagonal	Yes	No	No	No
main diagonal	Yes	No	No	No

As nouns, principal diagonal and main diagonal are synonyms defined as:

principal diagonal and main diagonal: the diagonal of a square matrix running from the upper left entry to the lower right entry

principal diagonal (noun)	main diagonal (noun)
the diagonal of a square matrix running from the upper left entry to the lower right entry	the diagonal of a square matrix running from the upper left entry to the lower right entry