What a community agrees to treat as value in exchange. Currency works only because everyone expects everyone else to accept it.
Latin currens (running, flowing), present participle of currere (to run); what a community agrees to treat as value in circulation, named for the running-through it does between hands.
Latin currere is to run, and currency was named for its motion, not its worth — value that keeps flowing from hand to hand rather than value that sits still. It stops being currency the moment holders stop expecting it to keep moving.