Psychic Paper is a tool utilized by The Doctor on the television show Doctor Who. It is a new introduction to the franchise, only being used in the revival. It was first shown in the episode "The End of the World".
The paper looks like a blank piece of paper, the size of a passport. Its main function is to psychically show whatever the viewer needs to see. The Doctor uses it to present whatever credentials he might need in a given situation. It is not perfect, since it doesn't work on certain aliens, psychics, or people who have been taught to detect it. It also humorously malfunctioned when the Doctor needed it to say he was a "mature and responsible adult", since that was too large of a lie for it.
It can also be used to carry messages, if the sender of the message has a psychic ability and desire strong enough to trigger it. River Song first contacts the Doctor from his perspective when one of her psychic messages to him reaches him at the wrong point in his timeline.
Like much on Doctor Who, especially its companion object the Sonic Screwdriver, the Psychic Paper is usually used to move the plot along in whatever way it needs to be moved, when it will save time and complication to get The Doctor into where he needs to be. And much like the Sonic Screwdriver, it the Psychic Paper is overused, or becomes too much of a handwave, it would ruin the drama and adventure of the show. However, the Psychic Paper is not used too much, coming up only every third episode or so.