>>illegal identifiers can be delimited with brackets:
>
>What is the definition of an illegal identifiers?

The regular Identifiers
- Can start with letter, underscore (_), "at" sign (@), or number sign (#).
- Can include letters, digits, "at" sign, dollar sign ($), number sign, or underscore
- Cannot be T-SQL reservered word

The rest are delimited Identifiers.
See Using Identifiers in BOL for details.