What term describes nonstandard grammatical usage that violates established grammatical rules?

The term that describes nonstandard grammatical usage that violates established grammatical rules is "solecism." This term specifically points to any deviation from the norms of standard grammar, whether such errors occur in terms of syntax, word usage, or even pronunciation. Solecisms can manifest in various forms, like mismatched verb tenses, incorrect word forms, or improper sentence structure. The concept underscores the importance of adhering to grammatical conventions in effective communication to ensure clarity and comprehension.

The other options refer to related but different concepts. A syntax error pertains to mistakes made in the arrangement of words and phrases in a sentence that disrupts grammatical correctness. Grammatical structure refers to the overall organization and framework of sentences as dictated by the rules of grammar but does not specifically denote errors. Language shift pertains to a change in language use within a community, often reflecting broader sociolinguistic trends, rather than focusing solely on individual grammatical inaccuracies.