In the Chrestomanci series
" is one of the classifications of magic-user recognised in World 12A
of the Related Worlds
It is a specialty classification, and isn't part of the grading scale that includes "enchanter
" and "witch
A Diviner uses magic to find things out - where a person is currently living, for instance, or what is stopping a spell from working.
Some diviners use crystal balls.
Dr Pawson is the most eminent Diviner in England; Christopher Chant is sent to him in The Lives of Christopher Chant so that he can find out why Christopher can't do magic.