A credit was a generic term for a monetary unit. Many worlds and civilisations described their money with the term. While they were not the same system, they often (but not always) referred to a currency system that operated electronically, rather than using currency (i.e., notes and coins).
Examples of monetary systems
The Stoish civilisation, for example, had credits which were stored on "Credit Cards'". The Tenth Doctor said that £1,000,000 was equal to 50,000,056 credits in the early 21st century, meaning that the average Sto credit was roughly equivalent to 2 pence or 3 cents. (DW: Voyage of the Damned)
Credits were used in various human societies.
- On Sto 5,000 phone calls to a contest costed 5,000 credits, roughly the same as the price for two 1st class tickets to a voyage with the spaceship Titanic. (DW: Voyage of the Damned)
- In the year 2050 credits (sometimes called "creds") were the currency in Department-controlled Britain. (K9TV: The Custodians, Oroborus) By 2062 the UK no longer used credits and had switched to the Euro. (NSA: The Last Dodo)
- In the 42nd century, in the Second Great and Bountiful Human Empire, the price of an Ood slave was 50 credits. (DW: Planet of the Ood)
- In the year 200,000, a Kronkburger onboard Satellite Five cost 2 credits 20. A Head Chip costed 100 credits and a full Infospike costed 10 000 credits. (DW: The Long Game)
- On New Earth, in 5,000,000,053, one patch of Forget 43 cost two credits. (DW: Gridlock)
The Inter Minorians and Lurmans used credit bars that were blue. One bar represented one credit. (DW: Carnival of Monsters)