Limiter

A "Limiter" (リミッター, Rimittā) is a term coined by Dr. Genus for his theory on explaining Saitama's seemingly limitless level of strength. According to his theory, there is a limit to which how strong a being can become or how much it can grow, placed upon him/her by God. This natural limit is so that an individual will not acquire so much power that he/she cannot control or handle it.

The knowledge of limiter is only known by Genus and Zombieman.

Breaking the Limiter
In rare instances, it is possible for certain individuals to remove their limiters. So far, there is only one confirmed individual who has managed to accomplish this feat: Saitama

Garou, another individual, had potentially removed his limiter during the showdown between the Hero and Monster Associations. It is stated that "Garou's limiter [...] seems to be breaking!" and he was shown signs of removing his limiter, such as rapid growth throughout his battles and a seemingly endless supply of energy; however, in the end, he started running out of fumes and began losing the power he had gained. Whether this means that he had failed to completely remove his limiter, or that he did but the limiter was placed back on, remains unclear.

The method or process of removing one's limiter are currently unknown, but it appears that there are side effects that vary from person to person (i.e. Saitama losing his hair). As Garou was breaking his limiter, he mentioned how his body felt light as if his "spirit" was removed from his body. Superalloy Darkshine used to lose his hair from extensive weight-lifting training but cannot remove his limiter like Saitama.

Becoming a Mysterious Being is not the same as breaking one's limiter. It is a mutation triggered by things such as pollution, injection of Monster Cells or a bad habit (or getting stuck in a bird's costume), and the individual is reborn as a new being with a new limiter.

Trivia

 * It is possible that the "God" Dr. Genus referred to is the same being that Homeless Emperor refers to as "God".
 * Garou's side effect for breaking his limiter is possibly a Mysterious Being mutation; however, it appears to be different somehow. Saitama didn't consider him to be an actual Mysterious Being, but rather just a human in a monster costume. Though Saitama's claim was false, unlike any Mysterious Being so far, Garou's transformation managed to revert. Saitama is also very aware that humans are capable of turning into Mysterious Beings (e.g. when Crabrante mentioned to Saitama that he transformed after eating too many crabs), so the fact that he saw Garou as a human after seeing his monster form hints a difference between Garou's transformation and an actual Mysterious Being transformation.