Individual schools of magic may have their own titles (such as Grand Abbie, the rank held by Janis) in addition to the generic designation. The relationship between such titles and the general proficiency level implied by "Master-class caster" is still unclear.
Mastermind is a term for Master-class Thinkamancer.
Any caster can gain Master-class by a caster gaining a powerful new insight into their discipline or major class. Book 2 Text Update 50