Used to give names to variables, functions, labels, defined types.
Consists of the 26 letters of the Latin alphabet (upper and lower case), plus the 10 western Arabic numerals, and the ASCII underscore character:
ABCDEFGHIJKLMNOPQRSTUVWXYZ abcdefghijklmonpqrstuvwxyz 0123456789 _
Cannot start with a number.
Cannot be any of the language's reserved words.