An Identifier is the name of some variable. When encountered it is looked up through the scope chain (10.3.1). The result of evaluating an Identifier is always of type Reference.
An Identifier may start with any letter(a-z) and numbers (0-9). The actual unicode ranges are a bit larger than that and include diacritical characters. They may also start with the "special" characters underscore (_) and dollar sign ($).
The remainder of the Identifier may also contain escape sequences to introduce a certain character. The result of such a variable will be a variable that's different from a variable that has the same character unescaped. Furthermore, escape sequences may not introduce characters otherwise illegal in an Identifier (like a line terminator).