An identifier is a name that is used to introduce a user-defined constant, variable or function. Identifiers consist of a sequence of letters (a b c..., A B C...), digits (1 2 3...) and underscores (_). The first character of an identifier must be a letter or an underscore.
Identifiers are case-sensitive, which means that
int Number, number;
would define two different integer variables. The maximum length of an identifier is 100 characters, and all of these are significant.