Arcane Rune

An Arcane Rune is the power of an Arcane Tome compressed into a single symbol.

Source
This item is a reward from the quest Synthesis of Power which begins with Voren'thal the Seer in the Scryer's Tier in Shattrath City.

Once that quest is completed, you can earn more Arcane Runes by completing the repeatable quest Arcane Tomes.

As currency
Arcane Runes can be used as currency with the Scryers faction to purchase shoulder armor enchants called inscriptions based on your reputation level: