(n.) one of the sets of three zeroes used to write out numbers like 1,000,000
You might have heard the word cipher being used in numerical contexts to refer to zero. In fact, when the word was first used in English in the fourteenth century that was its original meaning: cipher was initially a word for the concept, quantity, or symbol 0. In that sense it derives from an Arabic word, sifr, literally meaning empty (which is the origin, incidentally, of the French word for a numerical character, chiffre).
As an extension of that mathematical use of the word, cipher has also come to be used to refer to the trios of zeroes that are used to write out quantities like a million, 1,000,000:
So yes, even those have their own name. But there’s a big question unanswered here: how did cipher also come to refer to encrypted texts? Put another way, what does deciphering a code have to do with the concept of zero?
The short answer is that we’re not entirely sure. But the most commonly held theory is that the cryptographic sense of cipher stems from the use of numerals and other mathematical symbols (or, as perhaps was originally the case, shapes and invented symbols merely meant to resemble mathematical characters) in place of letters and words in encrypted texts.
The Oxford English Dictionary points out that one of the original traits of such texts was the “use of letters or characters in other than their ordinary sense”, so it’s tempting to imagine the similarity or overlap between the letter O and the numeral 0 might have been involved somewhere along the line here. As nice a theory as that is, however, problematically the concept of zero was originally represented by an interpunct, ( · ), not an O-like character—and by the time people began using the word cipher in relation to coded texts in the 1400s and 1500s, the word had broadened its meaning to any mathematical symbol, not just zero.
No matter whether actual letter–numeral confusion was involved or not, it seems the use of alphanumeric codes was behind the shift in meaning here. And as English adopted other words for the concept of zero (like—well, zero), there was no need for cipher to retain its original arithmetical meaning.