A Relic is an item in Borderlands 2 that passively boosts one or more aspects of a character. Though the abilities of relics are extremely varied, they are generally powerful bonuses to one or more related statistics, many of which can be increased by no other means.
A relic is different from an Artifact in that its use does not augment the user's Action Skill, is not restricted by class and may or may not be tied to a specific element. Relics appear only in Borderlands 2. All relics are made by Eridians.
Allegiance Relic - Boosts a specific manufacturer's effects (reducing recoil, increasing damage, accuracy, reload speed, etc.). Can have a maximum of two effects simultaneously; both are applied to the same manufacturer.
Stockpile Relic - Boosts the munitions a character can carry of a specific class of weapons. Can also increase the maximum number of grenades a character can carry. Maximum of three can be increased simultaneously.
Bone of the Ancients - Increases Damage of one elemental type and improves Action Skill Cooldown Rate.
Heart of the Ancients - Increases Damage from one specific weapon type, improves Fight for Your Life time and the HP gain from Second Wind.
Skin of the Ancients - Increases resistances of up to three elemental types, including non-elemental. Also improves shield Recharge Rate and/or Capacity.
Relics that boost stats by a variable percentage have their stat calculated by two variables. The first is the level of the relic and the second is the grade. The latter ranges between 7 to 15 and can result in obtaining a higher level relic that has worse stats than a lower level one. This also applies for Overpower levels, which mean the best version of the relic is usually obtained at the highest Overpower level at grade 15.