Sidereal means "starry," from Latin sidereus "starry, astral, of the constellations." The Astral World, the fifth dimension, related to emotion, the heart, the world of dreams, etc.
"...the Sidereal World, which is the Astral World." - Samael Aun Weor, Tarot and Kabbalah
"It is urgent to asseverate with great emphasis that the Sidereal Body or Astral Body of the Alchemists and Kabbalists is developed and splendidly unfolded under the absolute control of the twenty-four laws." - Samael Aun Weor, The Three Mountains