"Y Môr Iwerydd"

Translation: The Atlantic Ocean

[Esse-Honorem]

again, môr iwerydd, not cefnfor iwerydd. is it another familiar expression?

also, it seem like (from Wiktionary) that cefnfor is derived from cefn + môr

[EllisVaughan]

I personally would refer to the Atlantic ocean as cefnfor Iwerydd not Môr Iwerydd have you considered suggesting cefnfor Iwerydd as an alternative answer.

[Dim-ond-dysgwr]

So would I -- insofar as the WJEC's GCSE examination syllabuses (meysydd llafur arholiadau TGAU CBAC) consistently call the Atlantic Ocean "Cefnfor Iwerydd" -- with the only caveat being that many other sources (including https://cy.wikipedia.org/wiki/Cefnfor_yr_Iwerydd and https://cy.wiktionary.org/wiki/Cefnfor_yr_Iwerydd) name it as "Cefnfor yr Iwerydd".