Just off the cuff, as it applies to human beings, I would say the ability to acquire and retain knowledge and skills that are of benefit/of utility to an individual in navigating their environment and the situations they encounter, both external and internal, in service of survival, meeting their needs, and enjoying and taking pleasure in their life.
That plays out across multiple domains.
Self-awareness, reasoning, critical thinking, social skills, emotional skills, sexual awareness, risk assessment, abstraction, the ability to plan ahead, adaptability to changes, situationally-appropriate recall and efficacious application, logic, deduction, induction, creativity, problem-solving, error-correction...
Cheers,
Ian