Credit

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).

Human usage
In the year 2050 credits (sometimes called "creds") were the currency in Department-controlled Britain. (TV: The Custodians, Oroborus) By 2062 the UK no longer used credits and had switched to the Euro. (PROSE: The Last Dodo)

In the 42nd century (approximately year 4126), in the Second Great and Bountiful Human Empire, the price of an Ood slave was 50 credits. (TV: Planet of the Ood)

In the year 200,000, a Kronkburger on-board Satellite Five cost 2 credits 20. A Head Chip cost 100 credits and a full Infospike cost 10,000 credits. (TV: The Long Game)

On New Earth, in 5,000,000,053, one patch of Forget 43 cost two credits. (TV: Gridlock)

Other species
The Stoish civilisation 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. (TV: Voyage of the Damned)

The Inter Minorians and Lurmans used credit bars that were blue. One bar represented one credit. (TV: Carnival of Monsters)

A slave trade species in the future only accepted galactic credits as payment. (COMIC: Prisoners of Time)