Dragon Vein (竜脈 Ryūmyaku, alternatively "Dragon Pulse") is a special passage used by the Dragon Miko.
Z/X Encyclopedia Entry
The passage of the Dragon Miko that was stretched around to every place. She can instantly arrive in a destination no matter how far the distance is. If you lose your way here, you cannot get away without the guidance of Dragon Miko.
The Dragon Vein is based on the concept of ley lines, which also known as "dragon line".