From what I understand spell names are named in order to distinguish distinct spells and their variations.
  • Holy is a Light/Astral spell first used by the White Mages of Amdapor
    Ancient (spell name) is an older version of a spell used prior to the development of modern versions in the 4th Astral Era. Power seems to be slightly more powerful than the common versions.
  • Mega/Giga/Tera (spell name) are more powerful/scaled up versions of spells used by some Primals/Eikons and those they are empowering. Currently we have only seen versions of Flare (used by Bahamut Prime) and Holy (used by Alexander Prime).
  • Almost (spell name) are scaled down versions of the Mega spells and used by secondary creatures summoned by some Primals. Almost holy is the only version we have seen.
  • Holiest of Holies is a weaponized version of Flash using Light/Astral aether exclusively. Could technically be called Holy Flash.
  • Dark (Element) refers to distinct spells that admix Umbrally attuned aether with an Element. Stronger Voidsent tend to use various Dark Fire and Dark Blizzard spells.
  • Holy (Element) refers to distinct spells that admix Astrally attuned aether with an element. Many of the spells used by the Heavens Ward are this sort of spell.
  • (Beast tribe) (Element) are distinct versions of spells developed by members of various beast tribes. The Ixali's Aero series and the Moogles' Pom (Element) series are examples of these.