ESP is short for Extra Sensory Perception. It refers to any information gathered by means other than the five senses (or six if you want to count proprioception). Since, under current scientific theories, humans are only able to gain information through these senses, ESP is held in considerable doubt by scientists.

ESP and other 'Psychic Phenomena' have been the subject of a number of scientific experiments. Such studies even have a name: Parapsychology. Unfortunately, parapsychologists have been unable to come up with any proof that ESP exists or plausible explanation for how it could exist. Tests of psychic phenomena tend to become compromised, as can be expected when one is testing professional charlatans. Not only this, but the scientists tend to be overly credulous. This is probably because a skeptical person is likely to simply dismiss psychic phenomena, and very unlikely to make a career of testing them. This leads to scientists who are trying to get a positive result, and to compromised experiments.