Something that has not been adequately defined by some other definitions; refers to the rules used to combine words together in order to form meaning within a given language.
"i
don liek
u becoz u suck"==grammatically correct, despite spelling shortcuts and the lack of capitalization/punctuation. These are mechanical/technical errors.
"She go to house red last Saturday."==despite
perfect mechanics, this is not grammatically correct, due to a tense mismatch (needs to be past tense, verb misinflection (should be "she goes"), missing article ("the" or "a"), and adjective-noun word order reversal.