An identifier is a name that identifies an object or group of objects. The object or group of objects may be ideas, physical things or physical substances. An identifier can be a number, word, symbol, letter or mixture of those.
Numbers, words, symbols or letters follow an encoding system where they stand for ideas or lengthy names. Identifiers that follow an encoding system are called codes or ID codes. In computer science, identifiers name objects and are used in nearly all information processing systems. The case of alphabetic characters is important in programming because one word written three ways refers to three different things: "INDEX" is not the same as "index" or "InDeX."