William de Worde was his correspondent in Ankh Morpork.
The Roundworld equivalent is "Caliph".
A "Caliph" (pronounced khaleef in Arabic) is the head of state in a Caliphate, and the title for the leader of the Islamic Ummah, or global Islamic nation. It is a transliterated version of the Arabic word خليفة Khalīfah, which means "successor" or "representative".
Presumably Seriph is a cross between that and "Seraph" - a celestial being: the first of the nine orders of angels in medieval Christian angelology - as well as a reference to a seriph/serif, the typographical term for the flat bits that stick out at the end of the main line of a letter (this font is sans serif).