Second-language acquisition (SLA) is the process by which people learn a second language, and it is also the scientific discipline devoted to studying that process. Second language refers to any language learned in addition to a persons first language, and it can include the learning of third, fourth, or subsequent languages. Second-language acquisition assumes knowledge in a first language and encompasses the process an individual goes through as they learn the elements of a new language, such as vocabulary, phonological components, grammatical structures, and writing systems. Researchers define language acquisition into two categories: first-language acquisition and second-language acquisition. First-language acquisition is a universal process regardless of home language, while second-language acquisition is a more complex process that involves the transfer of knowledge from the first language to the second language. The field of second-language acquisition is regarded by some as a sub-discipline of applied linguistics but also receives research attention from a variety of other disciplines, such as psychology and education.