ESP stands for extra-sensory perception and is defined by the dictionary as the power to perceive things that are not present to the normal five senses. In other words, outside our normal terms of perception.
ESP is not really a psychic ability in its own right, rather it is a catch-all term for most psychic abilities. Clairaudience, clairvoyance, clairsentience, empathy, mediumship, reading and telepathy are all forms of extra-sensory perception. Any psychic (no matter what their particular gift was) would be termed to possess extra-sensory perception.