Someone with both a greater standard of all aspects of
grammar (than most people) and the willingness to uphold said standards in all that (s)he says and writes, including but not limited to speaking in
casual conversation and typing on the internet.
Grammar Nazis are not constrained to the internet.
Zealous
Grammar Nazis are also known to correct the
grammar of other people when they feel the need to, although the act of correcting grammar does not necessarily denote someone as a Grammar
Nazi. Someone who indulges in pointlessly correcting people, particularly nitpicking trivial mistakes, is more often a Grammar
Troll. That's something else entirely.
True members of the Grammar Nazi Party
will politely correct other people in a manner that does not shame the recipient and is always of mistakes that either change the meaning of the statement or are apparent that the recipient is ignorant of. Misuse of the apostrophe in "it's" and "its" and the misuse of homophones, however, are often corrected even if they do not apply to aforementioned standards.
As it has been mentioned that Grammar Nazis are not constrained to the internet, it should also be noted that Grammar Nazis interact in all languages that have codified rules of grammar (i.e. all official
world languages). Some fictional languages, such as Klingon (Star Trek) and the Ancient Language (The Inheritance Cycle) are also included.