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What is a grammatical norm?

I can't find a clear definition on Wikipedia or Google-searching, so please help if you have the exact from-the-book definition or at least something I could use. Thanks in advance!

EDIT: ctsmrvn, thank you for the answer, it has been rather helpful (since I don't have access to books that contain explanations and definitions of grammatical norm). The assignment I got was to write an essay on "A few examples of disorganization/disorder of Croatian grammatical norm" (the text in inverted comas is the title of an article given to me to write an essay about). Note - I'm studying for primary school teacher (in Croatia, obviously) with specialization in English language. I don't know anything about language structure on that level, my professor doesn't really give a damn about teaching, I passed the exam (and therefore the subject/course), but still I don't want to push my luck and fail if he asks me where's my essay when I go get my grade.
Thank you and please write some more, it'll help.

The concept of "grammatical norm" as linguists use the term will be difficult to find in online sources since it is extremely technical and involves a fairly thorough understanding of the analysis of language. You would like me to give you a single sentence explanation, but the only one I was able to locate is several paragraphs long, with lots of abstract examples and several references to works in French by Ferdinand de Saussure. Linguists generally think of a language as having three representations. The first is "what people are capable of saying." For example, English speakers are perfectly capable of saying "It's ten minutes before three," or "It's ten to three," or "It lacks ten minutes of being three o'clock," or "It won't be three o'clock for ten more minutes." In this first aspect, all of these sentences have equal value. The second is "what people generally recognize as the right choices." The third example above, "It lacks ten minutes of being three o'clock" follows the grammar rules of English, but we would find a number of speakers of that language who would not be comfortable with it. The third is what you are looking for. It's the "grammatical norm" which is the utterance or set of utterances most likely to occur. Examples one and two above (and a couple of additional ones, such as "It's ten before three") fit into this category. They are much more common. This is a summary of a much more technical explanation which occurs in "The Linguistics Student's Handbook" by Laurie Bauer, and while I normally do not do homework for others, this question is so technical and complex that only a linguist would really know. It is possible that the term "grammatical norm" means something else to the person who asked the question, so if you are not perfectly sure this is what is being asked, provide additional details about the question and the assignment.