The document discusses language aptitude, which refers to an individual's propensity for learning a second language. It describes key researchers in the field like John Carroll, who defined language aptitude and proposed it has four constituent abilities. Tests developed to measure aptitude include the Modern Language Aptitude Test (MLAT) and Pimsleur Language Aptitude Battery (PLAB), which assess abilities like sound discrimination and associating sounds with symbols. While intelligence relates to aptitude, researchers argue they are distinct factors in second language acquisition.