The Total Perspective Vortex (henceforth 'the TPV') is a device built as a practical application of the theory of atomic interactivity. The idea is that if every atom of the universe is affected by every other atom of the universe, then it is theoretically possible to extrapolate a model of the entire universe using a single piece of matter as a starting point. The TPV does this using a piece of fairy cake as its extrapolatory base.
Originally created by its inventor Trin Tragula as a way to get back at his wife (who always told him to get a "sense of proportion"), the TPV is now used as a torture and (in effect) killing device on the planet Frogstar. The prospective victim of the TPV is placed within a small chamber wherein is displayed a model of the entire universe - together with a microscopic dot bearing the legend "you are here". The sense of perspective thereby conveyed destroys the victim's mind.
The only person to survive the TPV is former President of the Galaxy Zaphod Beeblebrox. At the time he was exposed to the TPV, he was inside a computer-generated universe created for his protection by Zarniwoop. Since the entire universe was created for him, the TPV told him, in effect, that he was in fact the single most important person in the universe. This allowed Zaphod to survive the experience, and also did not surprise him in the least.
Zaphod proceeded to eat the fairy cake. This theoretically rendered the TPV inactive, until such time as a new piece of fairy cake could be delivered.