Imagine Breaker
Touma was born with the mysterious and logic defying ability known as Imagine Breaker. This ability allows Touma to negate all supernatural powers which include the effects of magical, psychic, and divine powers with physical contact of his right hand. Because of the passive nature of his ability, he is labeled as a “Level 0 Esper” for political purposes. Imagine Breaker can't be detected nor properly measured.
Although only his right hand can actively negate supernatural powers there are some exceptions. Anything supernatural that affects his body as a whole will be completely negated. This makes abilities such as teleportation ineffective on him. Touma has also shown immunity to telepathy despite it being an ability primarily focusing on the brain.
Not everything can be immediately dispelled by Imagine Breaker. If the supernatural force is sufficiently massive enough, it will take a few extra seconds before it's completely destroyed. More troublesome however are supernatural forces that can constantly regenerate. The only way to permanently negate a regenerative supernatural force is to destroy its source. Touma can fortunately counteract his disadvantage against sufficiently massive and/or regenerative supernatural forces by disrupting it. During its state of cancellation he can physically interact with it allowing him the ability to redirect it elsewhere. The strangest and rarest are supernatural forces that can only be dispelled within a very limited time frame. After that time limit has been passed that supernatural force is considered a natural phenomenon.
Supernatural forces aren't the only things Imagine Breaker can affect. Humans with supernatural powers can be nullified upon physical contact with his right hand. Their powers will return the moment he releases them. Supernatural beings respond differently to Imagine Breaker ranging from eradication, death, or banishment depending on what they are and feel subconsciously afraid of him. Supernatural constructs when touched lose their power and subsequently fall apart.
Imagine Breaker acts as a seal to an unknown entity or power. This "Invisible Thing" was able to terrify and overpower Fiamma of the Right, a magician that had briefly attained the position "La persona superiore a Dio" (The person superior to God), after he cut off Touma's right arm and used Imagine Breaker as a purification tool for his Holy Right. Touma himself was able to suppress that power and regrow his right arm. He regained Imagine Breaker while the one Fiamma disassembled into his body disappeared as only one could exist within the same world and its true power can only be wielded by Touma. Very little has been revealed about the "Invisible Thing" or its capabilities.
Index speculates that Imagine Breaker is the reason for his misfortune. Before he was sent to Academy City, Touma's misfortune wasn't just limited to himself. At a young age he was called "The God of Plague" and was ridiculed for how he seemed to bring misfortune to others. Sometime between him entering Academy City and when the series started this was limited to only himself. This may have something to do with the "Invisible Thing" sealed inside him.