grammar
gram-mar / ˈgra-mər

noun
1. the study of sentence structure
Grammar is to a writer what anatomy is to a sculptor, or the scales to a musician.
B.J. Chute, 1913-1987

2. the way in which sentences are constructed in a language
Writing is an act of faith, not a trick of grammar.
E.B. White, 1899-1985

3. the rules for how language is structured
Do not be surprised when those who ignore the rules of grammar also ignore the law. After all, the law is just so much grammar.
Robert Breault, 1963-

4. a style of speech or writing as it pertains to the rules for that language
Illiteracy at the poverty level (mainly a matter of bad grammar) does not alarm me nearly as much as the illiteracy of the well-to-do.
Mary McCarthy, 1912-1989

5. the principles of a science, art, etc.
Statistics is the grammar of science.
Karl Pearson, 1857-1936

Etymology
From the Latin translation grammatica ars (the art or science of letters) of the Greek equivalent grammatike [from gramma (letter)] techne. See the Middle English gramer and the Old French gramaire.
Thank you to Allen Ward for providing this etymology.