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 ranging from Level 0 to Level 5 by Academy City in an attempt to rank how powerful their ability is. However, not all who have gone through the program are capable of even attaining an ability; those that don't are ranked Level 0 by default. It is commented by Kamijou Touma in Volume 1 of the light novels that there is only 1 in 328,571 people who are Level 5s in Academy City's population of 2.3 million (not counting Misaka clones)- a total of seven Level 5s.

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

It has been stated that even those who have a Level 0 rank have enough esper ability to barely pass grading on the esper curriculum.