CSAT stands for Civil Services Aptitude Test in the context of the UPSC exam. It is the second paper in the preliminary examination and aims to assess the analytical skills, comprehension abilities, and decision-making skills of the candidates. Though it is qualifying in nature, a minimum score is necessary to advance to the main examination. The CSAT evaluates various competencies, including logical reasoning, mathematical skills, and general mental ability, which are vital for effective performance in civil services.