Someone who is obsessed with all things Japanese and anything related to Japanese things - to the point of being retardedly obsessive over Japanese things.
They believe Japan is superior to every single culture out there, will try to speak/learn Japanese, and their cultural understanding of Japan stems from the anime/manga/J-music/fashion that they follow - ultimately forming a distorted view of a country.
They will often comment on how every other culture (esp their own) sucks in comparison to Japan and how they wish they lived there.
Jtards completely misunderstands the culture they praise and make complete asses out of themselves.
A: "Wow the Japanese are so original and brilliant. I love their lolita fashion and FF music! We Americans are so stupid compared to them!"
B: (face palms) "You know FF music is Classical music which is Western in origin not to mention lolita clothing as well."