What is "onomancy"

onomancy \on"o*man`cy\, n. [Gr. 'o`noma name + -mancy. Cf. Nomancy.] Divination by the letters of a name; nomancy. [R.]
--Camden.

n. divination by the letters of a name; nomancy

n. divination by the letters of a name

Onomancy (haplological for Onomamancy or Onomatomancy) is divination based on a subject's given name, popular in the Late Middle Ages, usually following practices of Gematria.