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 he/she 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 break their limiters. So far, there are two individuals who have managed to accomplish this feat:
 * Saitama
 * Garou (formerly)

The method or process of doing so are currently unknown, but it appears that there are side effects that vary from person to person (i.e. Saitama losing his hair). After Garou removed 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. As shown through Saitama's battle with Personification Of A Light Pull Cord around one year after breaking his limiter, or Garou's battles with the Hero and Monster Associations, a broken limiter allows the individual to grow rapidly throughout a battle (but the individual still has the potential to be defeated, if the opponent is strong enough).

Becoming a Mysterious Being is not the same as breaking one's limiter. The 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 somehow. 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 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 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.
 * As his transformation reversed, Garou started losing the power gained by them. A potential cause for this is Garou's limiter being placed on him again, so had any excess power that he gained that he shouldn't have had began leaking out.