Limiter

A "Limiter" is a term coined by Dr. Genus for his theory on explaining Saitama's unnatural level of strength. According to the theory, there is a limit to which how strong an individual can become or how much can it 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.

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: Garou, another individual, had potentially removed his limiter during the showdown between the Hero and Monster Associations. In the webcomic, 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 started losing the power he had gained. Whether this means that he had failed to remove his limiter, or that he did but the limiter was placed back on, remains unclear.
 * Saitama

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.

Breaking one's limiter does not mean infinite power: instead, it means that the individual has no restrictions on the level of growth he/she can achieve.

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

Trivia

 * It is unknown if "God" that Dr. Genus referred to is the same being that Homeless Emperor refers to as "God".
 * Even without actual combat, Saitama managed to accumulate a ridiculous amount of strength by simply doing 100 push-ups, 100 sit-ups, 100 squats, and a 10 km run each day. After three years of this, he became so strong to the point that he can defeat almost any opponent with a single punch.
 * Garou's side effect for breaking his limiter is possibly a Mysterious Being mutation; however, it appears to be different. Saitama didn't consider him to be an actual Mysterious Being, but rather just a human in a monster costume. Though this was false, unlike any Mysterious Being so far, Garou's transformation reversed. Saitama is also very aware that humans are capable of turning into monsters (e.g. when Crabrante mentioned to Saitama that he transformed after eating to 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.