C - what is identifier

An identifier is used for any variable, function, data definition, etc. In the C programming language, an identifier is a combination of alphanumeric characters, the first being a letter of the alphabet or an underline, and the remaining being any letter of the alphabet, any numeric digit, or the underline. 

• The case of alphabetic characters is significant. Using INDEX for a variable name is not the same as using index and neither of them is the same as using InDeX for a variable name. All three refer to different variables.
• According to the ANSI-C standard, at least 31 significant characters can be used and will be considered significant by a conforming ANSI-C compiler. If more than 31 are used, all characters beyond the 31st may be ignored by any given compiler.