A secret or disguised way of writing, where the real message is hidden by changing the letters or symbols. It can also mean a code or, in older use, the number zero.
From Middle French "cifre," from Arabic "ṣifr" meaning "empty" or "zero." The idea of "zero" as an empty placeholder grew into the idea of something hidden or unreadable, leading to the meaning of a coded message.
The same root that gave us the idea of "zero" also gave us "cipher" for secret writing—both started as a kind of "nothing" that quietly controls everything. Without that "empty" symbol, modern math and modern encryption would both collapse.
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