Skill: Concentration
The Concentration is a relatively passive, but important skill for maintaining control of oneself, or pushing through certain ill effects. Concentration talents are often passive, or allow for easier saves during certain situations.
Mechanically, a creature can Concentrate⚙️ on a number of abilities (including spells) equal to their proficiency in the Concentrate skill, even untrained (e.g. A lvl 1 Wizard with 3 CON or 2 CON and proficiency in this skill (+1) can Concentrate on 3 spells).
Concentration only uses CON / Constitution by default. Concentration is a Primary Skill, meaning it is a major skill that can only be gained by mechanics that grant proficiency.