Money

Money, or cash, (TV: The Categories of Life) was a medium of exchange used to facilitate transactions of goods or services. (TV: The Man Who Never Was)

According to the Twelfth Doctor "Nothing is protected like money." because it was protected by the best firewalls in the universe. (TV: The Husbands of River Song)

The Sevateem had no concept of money. (AUDIO: Death-Dealer)