1. In communications and information processing, code is a system of rules to convert information—such as a letter, word, sound, image, or gesture—into another form or representation, sometimes shortened or secret, for communication through a channel or storage in a medium
2. Code is a system of words, letters, orsigns used to represent a message insecret form, or a system of numbers,letters, or signals used to representsomething in a shorter or more convenientform
https://en.wikipedia.org/wiki/Code (1.); http://dictionary.cambridge.org/dictionary/english/code (2.)