Esper

An Esper (能力者) is a person who has gone through the Power Curriculum Program and is theoretically capable of displaying a psychic ability; these individuals are given ratings of Level 0 through to Level 5 by Academy City in an attempt to rank how powerful their ability is. However, not all those who have gone through the program are capable of even attaining an ability, and are ranked as Level 0 by default. It is commented by Kamijou Touma there are only 1-in-328,571 people who are Level 5's in Academy City's 2.3 million people (a total of 7 Level 5's) in Volume 1 of the light novels (not counting Misaka clones).

Once a person becomes a esper they unconsciously emit an invisible energy field known as AIM, and are then incapable of using magic without inflicting harm to oneself.