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In communications, 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, a cable code replaces words (eg, ship, invoice, ...) into shorter words, allowing the same information to be sent with fewer characters, more quickly, and most important, less expensively. 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 words sent.

The term code is very often confused with the term cipher. These can be destinguished best by the purpose to use the code. A code is just a rule to represent information in another way - this doesn't necessarily imply secrecy. When the information should not be known to somebody else but the intended recipient, there has to be some secret knowledge not known to anybody else to decode it. A code which requires such secret knowledge (or is this secret knowledge) is a cipher. Thus a cipher is a special case of a code, since it is just means to represent the information in another way - with the intention, to keep it secret. However, it is more appropriate to call it cipher then. Historically there were codes for secrecy, which operated on word or phrase-level. These were called nomenclators.

Codes can be used for brevity. In the days when telegraph messages were the state of the art, elaborate commercial codes that encoded complete phrases into single words (typically 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?"), or AYYLU ("Not clearly coded, repeat more clearly."). The purpose of these codes was to save on cable costs. Nowadays there are codes for data compression.

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).

Codes may also be used to represent data in a way more resistant to errors in transmission or storage. Such a code is called Error-correcting code.

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. A cable code, of which only the sender and receipient have copies, would be one such.

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. '30' was widely used in journalism to mean end of story, and it is sometimes used in other contexts to signify 'the end'.

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".

Engineers often use the word "code" to mean a single program. They might say "I wrote a code" or "I have two codes". No software engineer or computer scientist would say that. They would say "I wrote some code" or "I have two programs".

A code is also a 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, particularly in cryptographic contexts. Use it with care.