制空圏- seiku ken; seiku gen

Means "empty sphere control" or "empty air sphere" or "mastery of the air system".
This concept has occurred a number of times in anime series, which does not make it any less valid.

The concept is simple - in any fight you establish a "zone of control" around yourself and then only defend that space. Any attack coming at you that does not enter this space you can ignore; any attack that enters this space must be countered.

Also, in any fight when attacking you must first move your opponent into this space or allow them to enter; to do otherwise gives your opponent an opportunity to read your intent and to counter you and also prevents you from defending your space, effectively breaking your barrier.