Accelerated Development

Accelerated Development is the ability to improve one's strength and skills at an extremely fast rate.

Overview
The user can grow far stronger through fighting and training, improving their natural abilities at a far more rapid pace than most other individuals.

Saitama
Nearly a year into his training regiment, Saitama noted that as a result of his repetitive routine, his body started adapting to "moving like a hero should", and his power began to explode at a rapid rate. In no time at all, he reached unparalleled levels of strength, speed, and durability through this regiment, a testament to his growth rate. However, according to Genos, Saitama himself cannot explain the secret behind his own strength; the cyborg being certain that he shouldn't have gotten as unbelievably powerful as he currently is by doing such a mundane level of training. Even after reaching an extreme level of power, to the point where nobody was remotely on par with his strength, Saitama continues to grow in power. After experiencing an upsurge of intense emotion, Saitama's rate of growth increase exponentially so that he rapidly outclassed Garou with every subsequent punch, despite Garou's attempts to copy Saitama's level of strength.

Garou
Garou's usage of this ability is also at an incredible degree, as he quickly gets stronger after each battle. His usage of this ability is enhanced even further after his transformation into a half-monster, going from having difficulty against Royal Ripper to effortlessly defeating him. During his fight with Orochi, Garou has shown to become stronger and faster the longer he fights, only being subdued when the Monster King decided to quickly knock him unconscious without giving him the chance to evolve further.

When fighting Superalloy Darkshine, an extremely powerful S-Class hero known for his remarkable strength and overwhelming durability, Garou initially was unable to even leave a mark on him and had difficulty keeping up with his strength. However, as their fight went on, Darkshine noted that not only was Garou becoming more and more powerful every time he was knocked down, but he also seemed to further improve his fighting skills and techniques in the middle of the fight, allowing him to completely overwhelm the hero. Later on, in a more evolved state, Garou has once again demonstrated his advanced growth rate in his battle against Platinum Sperm, going from exchanging numerous blows with him and seemingly only gaining a slight advantage, to being capable of defeating him in a single attack only moments later.

Glasses
Additionally, the B-Class hero, Glasses has demonstrated some degree of a heightened growth rate; although nowhere close to the same level as Saitama or Garou. During his time under the Blizzard Group, Glasses was an unknown and relatively weak B-Class, lacking any notable fighting skills or physical abilities. However, after leaving the group and setting off on his own, Glasses began training extensively; building up enough willpower to restart his entire hero career solo and deciding that he "had no limits". As a result of constantly ripping his entire body apart exercising, his physical strength and speed grew exponentially, allowing him to rise up to B-Class Rank 20 in record time; a feat which the B-Class Rank 2 Hero, Eyelashes, commented was "unbelievable". Despite this improvement however, Glasses noted that he still hadn't caught up to Eyelashes. Eventually, he grew strong enough to stagger and keep up with a weakened and exhausted, but just as formidable Garou; although, once again, Glasses admits that his teammates (A-Class Rank 10, Stinger and B-Class Rank 6, Wild Horn) were more suited to close quarters combat than he was.

Puri-Puri Prisoner
While not at the level of people like Garou, Puri-Puri-Prisoner showcases the capability to grow in strength and overall physical prowess at a rate much faster than normal humans can hope to achieve. While at the start of the series, he was defeated with only slight effort at the hands of the Deep Sea King, a Demon-level (albeit a particularly powerful one) Mysterious Being, since his training, he has grown much stronger than he was before. Most notably, he was able to defeat a monster as strong as the Deep Sea King without much effort, and he also later withstood multiple attacks from Dragon-Level monsters, albeit with heavy damage.

Pesky Clown
The longer he fights against a more capable opponent than himself, the more Pesky Clown adjusts and evolves in order to match them. Sweet Mask calls such a monster a "Rapid Growing" type.