A Rhetor is a legendary figure able to "change the past" through the manipulation of records and human memory. Very little about them is known or concretely recorded, which of course may be a result of their ability. To most in the Mathic world, the Rhetors, along with the Incanters were figures from Saecular legends about the Avout who arose prior to the Third Sack.
As the novel develops, it is revealed that Fraa Jad is a real Incanter who has the ability to perform Polycosmic Manipulation and it is speculated that after the Third Sack, certain Millenarian Maths (most likely the three inviolates) were allowed to retain such skills. While the Incanters were associated with the Halikaarnian orders, the Rhetors were associated with the Procians. In spite of earlier professed ignorance, Fraa Lodoghir as a leading Procian, knows about the skills of Fraa Jad and the inconsistent memories described at the end of the story, suggesting he may be a Rhetor, or that the Millenarians he is seen with at the end may be Rhetors. These may also be a result of Jad's actions as an Incanter, or some combination of the two. It is suggested that cooperation between then Procians and Halikaarnians has produced extraordinary results.
Exactly how Rhetor ability works is unrevealed, but it may be connected with the different memories various characters have of the Battle of the Daban Urnud. In the discussions of the polycosm, 'real' narratives are often distinguished from 'non-real' ones by whether they are consistent – a narrative with an ice block inside a star would be considered 'real', for example, but only if there were memories and records of why it's there. This may suggest that the Rhetor ability actually complements that of the Incanter.
The word is a play on Rhetoric. The two forms of discourse from Athenian Greece were Rhetoric, or writing, and Dialog, a discourse between two opposing viewpoints. Since it is possible to say what ever you want in Rhetoric, it is possible to Alter the past by telling a different set of events. The tension betwen the Rhetors and the Incanters is an extension of the conflict between the Socratic school and the Sophists.