ci·pher also cy·pher 
(sīfər)
(sīfər)n.
1. The mathematical symbol (0) denoting absence of quantity; zero.
2. An Arabic numeral or figure; a number.
3. One having no influence or value; a nonentity.
4.
a. A cryptographic system in which units of text of regular length, usually letters, are transposed or substituted according to a predetermined code.
b. The key to such a system.
c. A message written or transmitted in such a system.
5. A design combining or interweaving letters or initials; a monogram.
v. ci·phered, ci·pher·ing, ci·phers also cy·phered or cy·pher·ing or cy·phered
v.intr.
To solve problems in arithmetic; calculate.
v.tr.
1. To put in secret writing; encode.
2. To solve by means of arithmetic.
[Middle English cifre, from Old French, from Medieval Latin cifra, from Arabic ṣifr, from ṣafira, to be empty (translation of Sanskrit śūnyam, cipher, dot); see ṣpr1 in the Appendix of Semitic roots.]