1. Someone who is versed in the knowledge of grammar, or of language generally; a philologist; often signifying also a writer about, or a teacher of grammar who teaches the rules setting forth the current standard of usage for teaching or reference purposes..
2. Someone who is very skilled in grammar or a linguist who specializes in the study of grammar and syntax; such as, a set of rules that describe the structure of a language and control the way that sentences are formed.
3. A writer on grammar, especially someone who espouses (gives one's loyalty or support to) or advocates prescriptive rules.