1) A "TALMID CHAVER"
OPINIONS: Rav Ashi asked Ravina, his student, why one is permitted to render a Halachic decision without ascribing it to his mentor. Ravina answered that he is a Talmid Chaver of Rav Ashi, and a Talmid Chaver is permitted to render Halachic decisions when he is not in the presence of his mentor. This is also the reason why Rav Hamnuna rendered Halachic decisions when he was in a different city than his mentor, Rav Chisda (see Tosfos 62b, DH Rav Chisda).
What is the definition of a "Talmid Chaver"?
(a) RASHI (DH Talmid Chaver) explains that a Talmid Chaver is one who is just as knowledgeable as his mentor, but has learned from him "one or more" things.
(b) The RAMBAM (Hilchos Talmud Torah 5:5) implies that one who has not learned most of his knowledge from a certain teacher is considered a Talmid Chaver, even though he is not on an equal level with him in Torah knowledge. (The RIVASH (#271) writes that when Rashi, too, defines a Talmid Chaver as one who is equally wise, he does not mean it literally, but rather he means that the mentor is not the primary source of the Talmid's knowledge, as the Rambam explains. The Rivash proves this from Rav Hamnuna and Ravina, the Amora'im mentioned in the Gemara here, each of whom was called a Talmid Chaver but was certainly not equal in wisdom to Rav Chisda and Rav Ashi, respectively.)
(c) The MAHARIK (#170, cited by the REMA YD 242:7) writes that a Talmid Chaver is one who began learning from his mentor as a Talmid, but he grew in his Torah wisdom until he became as great (or almost as great, according to some) as his mentor. This is also the explanation of the RASHBAM in Bava Basra (158b, DH Talmid Chaver). (According to the Maharik's explanation, it is possible for a full-fledged Talmid to become a Talmid Chaver.)
(d) RABEINU SHLOMO SIRILIYO (cited by the YAD MALACHI) defines a Talmid Chaver in the opposite way from the Maharik's definition. A Talmid Chaver is one who was on an equal level with his colleague in Torah wisdom and was then surpassed by his colleague. That is, he started as a "Chaver" and then became a "Talmid." (The Yad Malachi, however, presents a number of proofs which refute this explanation.)
2) HALACHAH: "MOREH HALACHAH BIFNEI RABO"
QUESTION: Rava states that a student who pronounces a Halachic ruling (on an issue that is not straightforward) in the presence of his Rebbi ("Moreh Halachah Bifnei Rabo") is Chayav Misah. If his mentor is not present, it is forbidden, but the student is not Chayav Misah. The Gemara implies that "Lo b'Fanav," not in front of his mentor, means that the Talmid is more than three Parsa'os (twelve Mil) away from his mentor, and yet he is still forbidden to issue a Halachic ruling.
How is Rava's ruling to be reconciled with the Gemara earlier (62b) that says that Rav Chisda ruled in Kafri in the days of Rav Huna, his mentor, and Rav Hamnuna ruled in Charta d'Argaz in the days of Rav Chisda, his mentor? If one is forbidden to be Moreh Halachah even when not in front of his mentor, then why did they do so?
(a) TOSFOS (DH Rav Chisda) answers that Rav Chisda was a Talmid Chaver of Rav Huna, and Rav Hamnuna was a Talmid Chaver of Rav Chisda, and that is why they were permitted to be Moreh Halachah when they were not in front of their mentors. In front of one's mentor, though, it is forbidden even for a Talmid Chaver to be Moreh Halachah.
(b) TOSFOS in Sanhedrin (5b, DH Ela Im Ken) answers that a student is permitted to issue a Halachic ruling while not in the presence of his mentor when he is more than three Parsa'os away, but only on condition that he has permission from his mentor to do so. When the Gemara here says that it is forbidden even not in the presence of his mentor, it refers to when the student does not have permission from his mentor.
(c) The RAMBAM (Hilchos Talmud Torah 5:3) answers that one is forbidden to issue Halachic rulings even not in the presence of his mentor only when he regularly does so. A student is permitted, though, to issue occasional rulings when he is not in the presence of his mentor.
(d) The RASHBA and RAN explain that when the Gemara says that "she'Lo b'Fanav" is forbidden, it means that the student is within three Parsa'os but not directly in front of his mentor. If he is more than three Parsa'os away, then even a lowly student is permitted to issue Halachic rulings. (See Chart.)