Most strictly defined, an alphanumeric character is any element in the set that includes the upper and lower case alphabetic letters (A-Z, a-z) and decimal numerals (0-9). It is useful to group these letters and numerals together because many computer programs treat them differently from punctuation characters. For example, some operating systems allow all the alphanumeric characters in filenames, but prevent the use of certain punctuation characters. Some file-naming protocols, however, do expand the definition of alphanumeric characters to include additional symbols; for example, International Business Machines (IBM) mainframe computers treat @, #, and $ as alphanumeric characters.

With reference to general computer input and output, the alphanumeric set generally includes the upper and lower case alphabetic letters, the numeric characters, and all the various special characters (symbols such as @, #, $, +, <, and =). An alphanumeric character set may also include letters from non-English alphabets, such as é or ç. The set of alphanumeric characters used for input and output by a specific type of computer is called an alphanumeric code. Two well-known alphanumeric codes are the EBCDIC and ASCII codes. Extended Binary Coded Decimal Interchange Code (EBCDIC) is an IBM code that uses 8 bits to represent 256 possible characters. EBCDIC is used mostly in IBM mainframes and minicomputers. The American Standard Code for Information Interchange (ASCII) is a code that uses 7 or 8 bits to assign numeric values to up to 256 unique characters. ASCII is used with smaller computer systems, such as personal computers.