In the Harry Potter books, the pensieve is a magical device that allows people to store their own or other's memories, and later relive and witness them at their leisure. The pensieve looks something like a birdbath, being a basin on top of a pedestal. Inside the basin, memories and thoughts (depicted as a swirling silver liquid), are deposited and received. A person touching the pensieve is immersed in the memories contained within it. Pensieves seem to be somewhat rare devices, since only one, belonging to Albus Dumbledore, is shown in the books. The pensieve is first introduced in the fourth book, and is used in each of the following books. The name is derived from "pensive", meaning "to think over".

J.K. Rowling wrote a series of books that were so engaging that it was sometimes easy for readers to overlook some of the flaws in what she was doing. One of her greatest weaknesses was her dependence on infodumps, long passages where the background of the story had to be explained. The pensieve is an antidote to this in some ways: whoever is witnessing the memories in the pensieve, and the reader, is shown, not told whatever information needs to be passed on to the character, and reader. Given the increasing complexity of the story, it would be hard to demonstrate all the background information without the use of the pensieve.