What do we call the set of rules that defines correctly structured documents or fragments in a formal language?

The set of rules that defines correctly structured documents or fragments in a formal language is referred to as syntax. In programming and formal languages, syntax specifies how symbols can be combined to form valid statements, expressions, or programs. It is akin to the rules of grammar in human language, dictating how sentences are constructed.

In the context of computer science and programming, syntax is essential because it ensures that the code is written in a way that the compiler or interpreter can understand. If the syntax is incorrect, the code will not execute, leading to errors.

While grammar can sometimes refer to the overall structure and rule system of a language (and is often used interchangeably with syntax in informal discussions), the term "syntax" is more specifically focused on the rules governing the combination of symbols. Semantics, on the other hand, relates to the meaning of the statements formed by the syntax, not the structure itself. Style pertains to the aesthetic choices and formatting of code rather than its structural correctness. Thus, the correct answer emphasizes the importance of structured rules in formal languages.