Either innate natural ability related to one area or another, or else ability to easily take in information (learn), process that information, and put that information to practical use (if/when needed) related to a specific area/subject.
Aptitude is your ability to learn ..... an aptitude test tests how you grasp concepts and how teachable you are. Top 3 definitions from dictionary.com below.
n 1. capability; ability; innate or acquired capacity for something; talent: She has a special aptitude for mathematics. 2. readiness or quickness in learning; intelligence: He was placed in honors classes because of his general aptitude. 3. the state or quality of being apt; special fitness.