Throughout the Bavli, but (based on a Sefaria search) never in the Mishnah, the Tosefta, or the Yerushalmi, we find the phrase "amar Mar" (אמר מר) without any other identifier (in contrast to "Mar Ukva," "Mar bar bar Ashi," and so on). Amar Mar is translated by Steinsalz as "The Master said." Who is this "Mar"/"Master"? Is there a "stam Mar" the same as there is a "stam Rebbi"? Or could it refer to multiple people? To further confuse matters (or me), sages named "Mar [X]" seem to always be amoraim, but at least in Chullin 94b:2, מר seems to refer to a tanna:
אמר מר ובעובד כוכבים בין שלימה בין חתוכה אינו צריך ליטול הימנה גיד הנשה במאי עסקינן אילימא במקום שמכריזין חתוכה אמאי אינו צריך ליטול הימנה גיד הנשה כיון דלא אכרוז אתי למיזבן מיניה
The Master said in the baraita: And in the case of one who sends a thigh to a gentile, regardless of whether it is cut or whole he is not required to remove the sciatic nerve from it. [etc.]
https://www.sefaria.org/Chullin.94b.2?lang=bi
First timer here, so please be gentle :)