I guess he is a Jedi but how can he be a "Jedi Master" which seems to be the requirement to train others?

@AdamGent - A Jedi "Master" is simply a Jedi Knight who's trained another Jedi to knighthood.

Ohh....... So a knight must train someone to become a master. That is you don't have to become a master to train. For some reason I thought that was the case with Anakin.

@AdamGent - Anakin was assigned Ahsoka while still a Jedi Knight. Had her training been completed (by her taking the Trials, he would have attained the rank of Jedi Master.

@Valorum aren't the only Jedi Masters those who sit on the Council?

@costrom - That's a pretty good question (and probably one that needs asking on the main site). The short answer is no, there are a bunch of Jedi Masters that don't serve on the Council (including, for example, Qui-Gon Jinn). I suspect your confusion comes from the fact that only Masters sit on the council, hence Anakin's upset.

@costrom: in ROTS, Anakin was appointed to the council without being granted the rank of Master.

@AdamGent I wonder if you could make the argument that Luke "trained" Vader in the last few minutes of his life.

@CortAmmon well in my opinion "Return of the Jedi" could also be interpreted the returning of Anakin to the light side (regardless of whether Luke is a Jedi or not since Jedi can be the plural form).

@AdamGent - "Who is the eponymous Jedi in Return of the Jedi?"

Not trying to discredit an otherwise great answer, but the argument for the Emperor's belief that Luke is a Jedi is arguable. He says it in response to Luke's claim that he is a Jedi, which means that the Emperor may simply be repeating Luke's claim without necessarily believing it to be correct.

It should be noted that the novelization is not part of the new canon, especially when the question concerns the sequel trilogy.

@Flater - True. It supplements the answer rather than answering it on its own

@AugustJanse - According to LucasFilm, novelisations are considered canon where they agree with what we see on screen.

@CortAmmon it is plural as far as I know. The german translation is "Rückkehr der Jedi Ritter", a plural expression