In communication, a code is a rule for converting a piece of information (for example, a letter, word, or phrase) into another object or action, not necessarily of the same sort. One reason for this is to enable communication in places where ordinary spoken or written language is difficult or impossible. For example, Morse code converts patterns of dots and dashes typed on a telegraph key into letters of the alphabet, allowing text to be sent over telegraph wires, flashing lights, or other channels that cannot carry speech or written text directly. Another example is the use of semaphore flags, where the configuration of flags held by a signaller or the arms of a semaphore tower encodes the message. Another person standing a great distance away can interpret the flags and reproduce the letters, enabling the signaller to communicate with someone too far away to hear ordinary speech.
Codes can be used for brevity. In the days when Morse code was widely used, elaborate commercial codes that encoded complete phrases into single words (five-letter groups) were developed, so that telegraphers became conversant with such "words" as BYOXO ("Are you trying to crawl out of it?"), LIOUY ("Why do you not answer my question?"), and AYYLU ("Not clearly coded, repeat more clearly."). The purpose of these codes was to save on cable costs.
Probably the most widely known communications code in use today is ASCII. It is employed by nearly all personal computers, terminals, printers, and other communication equipment. It represents 128 characters with seven-bit binary numbers--that is, as a string of seven 1s and 0s. In ASCII a lowercase "a" is always 1100001, an uppercase "A" always 1000001, and so on. Extensions to ASCII have included 8-bit characters (for letters of European languages and such things as card suit symbols), and in fullest flowering have included glyphs from essentially all of the world's writing systems (see Unicode).
Another use of codes is cryptography, where a code or cipher is used to disguise a message, preventing those not in on the secret from understanding a transmission. One simple method is to use a "codebook" with a list of common phrases replaced by phrases with completely different meanings, so that people without the codebook who may intercept the message hear a conversation that is entirely unrelated to the real intended message.
Acronyms and abbreviations can be considered codes, and in a sense all languages and writing systems are codes for human thought. Occasionally a code word achieves an independent existence (and meaning) while the original equivalent phrase is forgotten or at least no longer has the precise meaning attributed to the code word. For example, the number "86" was once used as a code word in restaurants meaning "We're out of the requested item". It is now commonly used to mean the removal or destruction of something.
In computer programming, the word code refers to instructions to a computer in a programming language. In this usage, the noun "code" typically stands for source code, and the verb "to code" means to write source code, to program. This usage may have originated when the first symbolic languages were developed and were punched onto cards as "codes".
A code is a also rule or a set of rules, such as code of honor, code of laws, or dress code.
This word has acquired a large number of subtly and grossly incompatible meanings. Use with care.