Spoiler Alert: This article/section contains details about future plotlines described in the light novels, not covered in the anime. Do not proceed unless you want to be spoiled.
Astral Dress (神威霊装,Shin'i Reisō?) or raiment is the Spirits' armor or clothing created by their power. Spirits can change it into whatever shape, color, style, or appearance they want. They seem to be extraordinarily durable, sustaining both the Anti-Spirit Team's as well as DEM'sRealizer's attacks.
The kanji that corresponds to the Astral Dresses in the light novel is formed by 神威 (Shin'i,神威?), which means "god's authority", while 霊装 (Reisō,霊装?) literally means Spirit's dress, which bears the meaning of "Spiritual Raiment of God's Authority" as a whole.
All of the Astral Dresses used by Spirits are named after the divine names of God, representing each respective Sephirah in the Kabbalah.
Although there are some discrepancies, as the names of God from Sephiroth positions 1–10 and their meanings are actually:
Ehyeh/Ehyeh Asher Ehyeh "I Will Be What I Will Be"
Yah "The Lord"
Yahweh Elohim "The Lord God"
Elohim Gibor "God is Hero"
Yahweh Tzaboath "Lord of Hosts"
Elohim Tzaboath "God of Hosts"
Shaddai El-Chai "The Almighty Living God"
Adonai ha-Aretz "The Lord of the Earth"
Part of this can be explained by the fact that according to Jewish tradition "Yahweh" (the 'true' name of God) is never spoken out loud and "Adonai" ('lord' or 'my lord') is always said in its place.
Unlike Angels, Astral Dresses do not possess an Inverse Form counterpart and simply change in appearance.
It is also currently unknown if they keep the same name or are another Astral Dress entirely.