air: Gilgamesh's weapon also known as the Sword of Disintegration or the Sword that Breaks the World apart. It's a powerful weapon that allows Gilgamesh to perform his Noble Phantasms.
aligment: The elemental standing of a person, object, place. Aligment strongly influente a Magi, since they affect their abilities through elements. Each Magi could have one or more leading element he/she is specialized in. Aligments could be changed but the practice is dangerous and somehow painful.
angra mainyu: in Zoroastrianism it's the concept of absolute evil, also known as Ahriman, associated with the evil spirit of the god Ahura Mazda. It's also the evil essence that taints the Holy Grail in the 3rd war and the one Kotomine Kirei wants to free, to learn the real nature of evil itself. In Fate/stay night it's represented as a sort of black fluid which spread and attacks its target devouring them immediately or tainting their body causing different effects (it depends on the person's will, purness or simple strenght...in Kiritsugu's case it just caused an illness which lasted several years before killing him). It's exterminated at the end of Heavens Feel.
archer: one of the servant classes. The archer is literally a unit/soldier using a long or short rage arc as weapon. The archer must have a good sight, precision and sence of distances as main features. Fate/stay night features two archers among its characters: Archer and Gilgamesh.
* This particular archer class could be mostly identified as the Ranger one in standard Rpgs systems.
argon coin: an artifact belonging to Caster which can summon the golden dragon Argon (or well the protector of the artifact itself).
astral plane: The dimension/plane of astral energy, or well where souls gather and merge.
assassin: one of the servant classes. The assassin is generally someone who kills someone else for mean reasons and in a very brutal way. Fate/stay night features two assassins among its characters: Sasaki Kojirou and Hassan Sabbah.
association: The Association of magician featured in all the TypeMoon games. It's seat is located in London and it has the main function to control the usage of magic throughout the world. It is also a sort of academy in which mages can improve their powers and skills. The Association is strongly against the Holy Grail War.
avalon (All is a Distant Utopia): Excalibur's sheats and also Saber's 3rd Noble Phantasm. According to the Arthurian legends it's told it was made by the faries and embellished with runes and druid spells in order to better protect the sword. It is also said that it can give the power of Immortality to its owner. In Fate/stay night we find out that it's implanted in Shirou's body and it's the reason why he has an incredible regenerating factor (and it's probably the reason why he summoned Saber).
avenger: one of the servant classes who appeared in the 3rd Holy Grail War and also in Hollow Ataraxia. Avenger is a weak servant who can only use knives as weapons and who has the ability to copy people's appearences and emotions. Fate/stay night features only one avenger who's a common indian villager, sacrified for being considered the evil Angra Manyu
avesta: an associable Noble Phantasm that can give the ability to record facts and events in an extremely accurate way. It's said it was used to release Angra Manyu in the 3rd Holy Grail War (mainly because it is strictly associated to its own background being a sacred text of Zoroastrianism).
azoth: the knife Kotomine Kirei gave to Rin as gift for the her anniversary for being a mage. It's also the weapon that kills him in Fate.
berserker: one of the servant classes. The berserker is most commonly someone who's taken by rage and who can't control himself anymore in that particular status. It's also said that people can fall into berserk if they have a strong spiritual factor and they can get into the control of the fury spirits. Fate/stay night features only one berserker among its characters: Heracles.
black saber: Saber's transformation into a Dark servant after being devoured by the Shadow.
blood fort andromeda (Outer-Seal . Temple of Blood): Rider's 3rd Noble Phantasm. Thanks to this ability Rider is able to cast a barrier of blood around her enemies, adsorbing their mana and energies and refurnishing herself. This is the same technique Rider casts on Shirou's school to gather mana in order to summon Pegasus.
burial cloth: A definition that could take its origins from the Sindone. The Burial Cloth as considered in Fate/stay night is normally the cloth wrapped around the body of a saint. There are several clothes featured in the story, such as the Martin Cloth (used by Shirou to supress the mana energy of Archer's arm) or the Magdalene Cloth (belonging to Karen, which is unbreakable by men).
caladbolg: Archer's Fake Spiral Sword used by him also as an arrow, and copy of the original Caladbolg belonging to Fergus mac Róich (character of the Ulster Tradition). It is said to have the power to slice the tops off hills and to be as long and bright as a rainbow. It's also related to the Gae Bulg of Cúchulainn and able to beat it. In other ways it's the inspiration for Excalibur (as a linguistic source).
caliburn: the supposed well-known Sword in the stone or as well an early transcription of Excalibur. It's the symbol of King Arthur's power over Britain and it's right to rule the kingdom being a descendant of the Pendragon. It's said this weapon was lost and in Fate/stay night it's the sword Shirou sees in his dreams and he eventually traces in Fate.
caster: one of the servant classes. The caster is a mage, wizard who's able to attack with magic spells or create magical creatures to use in battles (like golems and so on). Fate/stay night features one caster among its characters: Medea.
* The caster class can be considered as comprehensive of the standard magician classes of Rpgs systems.