Oyster card

An Oyster card was what Christina Da Souza referred to as "that card paying device thing" — an electronic card used to pay for bus trips in London. Whereas Christina was forced to give the bus driver her diamond earrings for her trip when she didn't have a card, the Tenth Doctor simply used his psychic paper in its place. Christina claimed desperately that she "only used her Oyster when there's an 'R' in the month;" it was April. (TV: Planet of the Dead)

When Lieutenant Doyle asked him for identification, the Sixth Doctor rummaged through his pockets and found many things, including an Oyster card. He exclaimed, "What on Earth's an Oyster card?!" (AUDIO: The Reaping)