The imagine breaker is the mysterious power that currently resides within Touma's right hand, allowing him to negate anything supernatural such as esper powers, magic powers, and (according to his own claims) God's miracles. More specifically, it is the power that can only exist in the right hand of Kamijou Touma due to "Imagine Breaker" finding meaning in being inside Touma's right hand. Neither magic nor esper ability, nor Gemstone-derived, the Imagine Breaker has long been under intense speculation both in-universe and in the fandom, with only a few people in-universe such as Aleister Crowley knowing fully of what it entails.
As shown throughout the series, Touma's Imagine Breaker negates both esper powers and magic. Moreover, in certain circumstances, it can negate even divine powers, such as the miracles of God (according to himself) and, more prominently, in his defeat of Archangel Gabriel in World War III.
In two different instances, Touma has demonstrated that he can dispel to an extent the effects of an area-wide attack. One instance is blocking the effects of electricity Misaka Mikoto created, by putting his right hand out to act as a lightning rod for the electricity, so it hits his right hand instead to any another part of his body. The other one is blocking a portion of Kaitabi Hatsuya's Graviton bomb from hitting the people behind him using Imagine Breaker as a shield. Nonetheless, it is required for him to have physical contact to initiate the effects of his power.
It seems that Imagine Breaker has a permanent canceling effect on some supernatural objects, as seen when it robbed Index's clothes of their Walking Church. Imagine Breaker can even ignore the actual weight or force of an object being propelled by supernatural power, as seen when Touma blocked a full-powered punch from a massive golem without showing signs of obvious effort, followed by the golem collapsing due to the magic that manifested it being abolished by Imagine Breaker. He did, however, "almost break his hand" in said exchange. Another example is when Touma blocked Acqua of the Back's magic-enhanced attack while in a weakened state, in contrast to how normal attacks (with no magic behind them) from Acqua could easily overpower him.