Hi - fire
  Jagan - Hiei's third eye, enabling him to see things no ordinary being can
  Katana - sword
  Ki - literally means the spiritual power of a person
  Kitsune - fox; pertains to Shuuichi Minamino's othe identity
  Koorime - ice maidens like Yukina
  Ningen - humans
  Ningenkai - human world
  Makai - world of the demons
  Rei - spirit
  Rei Gan - Spirit Gun
  Reikai - Spirit World
  Rei Ken - Spirit Sword
  Seiyuu - Japanese Voice Actors
  Youkai - beasts
  Yuyu Hakusho - Ghost files; Ghostfighter; Poltergeist Report; all are related

Used by Fans:

  Doujinshi/DJs - manga-like drawings made by fans
  Bishounen - male anime characters that resemble females. An example is Kurama
  Bishoujo - pretty female anime characters
  SD - stands for Super Deformed; these are anime characters in their little, deformed 
     forms
  Yaoi - male to male relationship can be found in some anime (certified by the creator) 
      and often in fanfictions (stories created by fans) which are a figment of their 
      imagination
  Yuri - the opposite of Yaoi; female to female relationship; can be found in some anime 
      (which are also approved by the creator) and often found in fanfictions for anime with 
      a lot of female characters