Which term is used for the rules that determine if a string of words forms a valid sentence?

The concept being referred to in this question is syntax, which is the set of rules that govern the structure of sentences in a given language. Syntax defines how words and phrases combine to create well-formed sentences, ensuring clarity and coherence in communication. In programming and many formal systems, syntax is equally important as it provides a structure that allows the language to be interpreted correctly by the compiler or interpreter.

While the term 'grammar' is also related to the rules of sentence formation, it encompasses a broader range of elements, including syntax, semantics, and more. Thus, while both grammar and syntax refer to aspects of sentence structure, syntax specifically pertains to the arrangement and order of words, making it the precise term regarding the rules for constructing valid sentences. In contrast, lexis relates to the vocabulary of a language, and semantics concerns the meaning behind the words and sentences.