Overskill

An overskill (口伝オーバースキル), is the capability of using one's own abilities, after the Catastrophe, in a way that was not intended by the game of Elder Tale.

Ga-tan calls it the "Skill of the Giants"; given that she is native to the world of Theldesia, it is possible that Overskills are something that already existed.

Overview
Overskills are discovered through experimentation, but are also often difficult to master without training and research. Potentially, these skills can be categorized as Continent-level or even Global-level Magic, depending on how they are applied and what impact they have on the world of Elder Tale. Overskills such as Shiroe's Contract Art Ceremony or Magical Engineering have already caused societal upheavals among the People of the Land.

It seems as though combat-usable Overskills create an entirely new skill in the system and, when discovered, prompt a screen to appear saying what skill they had created.

Trivia

 * According to one of Mamare's Twitter notes, Real Food Preparation creates food that can cause weight gain, unlike menu-crafted food.
 * The Japanese kanji name for Overskills, 口伝 (kuden), follows the naming pattern of regular Skills, and is the uppermost skill level in martial arts where the school's secrets and best techniques are only passed down orally to keep them from spreading.