Anything that seems to be bigger on the inside. Ref: The Doctor's TARDIS, which, while from the outside looks to be a simple phone box, actually is a time-and-space-traveling ship which includes but is not limited to a control room, a wardrobe room and a swimming pool.
Guy, watching incredulously as a Girl rummages in purse, digging out a wallet, a book, two tubes of lipstick, several hair accessories and a phone: "All that stuff fit in your tiny pocketbook?"
Girl: "Yeah. Time Lord Technology"
Guy: "Oh right...bigger on the inside"