Fujiwara Zone (藤原ゾーン Fujiwara Zon) is Takumi Fujiwara's technique as described by various characters, notably later on in the series, as they attempt to explain Takumi's ability to handle his AE86 against all odds. It wasn't until his race against Okuyama that the term was coined (though the term itself can be used to explain retrospectively almost all of the skills that Takumi has displayed in his races).
One of Takumi's oft-stated strengths is his incredible bond with the AE86, and how he drives it as though it were an extension of his body. It is also likely that Takumi has the mental control it takes to enter this "zone" almost as soon as he hits the throttle, and that his skill and equipment are both at a level where this state can be maintained indefinitely.
Fujiwara Zone has been variously explained as, or may be construed as a whole package that comprises: A near-supernatural ability to accelerate an FR car like a 4WD, an ability to change the rhythm of the racing line, and an ability to surprise his opponents by learning opponents' tricks on-the-fly during a race, among others. Fujiwara Zone may also be used to explain his ability to "race without seeing", to instinctively know after driving a course once, where the turns and tricks are, and can even be used to explain his being able to perform his trademark attack where he turns his headlights off and "disappear".