"The Master" — Eruvin 63a
תלמוד בבלי מסכת עירובין דף סג/א
רבינא סר סכינא בבבל אמר ליה רב אשי מאי טעמא עבד מר הכי
Generally, the words אמר מר — The Master said — precede the Gemara's analysis of the statement of a Tanna. Yad Malachi (Aleph §73) inquires as to whether we ever find the honorific Mar associated with an Amora. Halichos Eli (§83) notes that there is a case (Avodah Zarah 50a) in which R' Yochanan is afforded that title, but that may be an exception, as R' Yochanan is considered by some authorities to have had the status of a Tanna. He does note, however, that we do find Rav Huna addressed as Mar. The Yad Malachi himself notes that in Rosh HaShanah (20b) we find that R' Zeira is addressed as Mar, and he proceeds to cite other instances in which Amoraim received the title. (He also notes that elsewhere he has demonstrated that R' Yochanan does not have the status of a Tanna.)
In his notes to this Klal, the Yad Malachi cites other cases in which the honorific is given to Amoraim. So who does get the honorific? He cites a gloss to the Peirush HaMishnayos of the Rambam that Mar is always the title given to the Rebbe in a Rebbe-Talmid dialogue. However, asserts the Yad Malachi, our Gemara contradicts that position, as here Rav Ashi— who was senior — addresses Ravina— who was junior — as Mar. (He brings other proofs to contradict the aforementioned gloss.) Evidently, then, Mar is used inconsistently, at the discretion of the one person addressing another person.